feat(db): v5.3.0 inline databases, predefined templates & property validation

- Slash command /database (DATA group) -> template picker -> inline DB
  embed block rendered by FlowDeckDB. 'Get Started > Database' uses it too.
- 6 seeded database templates (CRM, Project tracker, Task list, Content
  calendar, Meeting notes, Reading list) with icon + schema; new service
  app/services/db_templates.py (materialize_properties, create_from_template).
- POST /db/api and /db/inline/api accept 'template' and materialize
  collection_properties. database_templates gets an icon column (migration v4).
- Property validation: collection_properties.validation_json (migration v4);
  validate_property_rule() in property_types (required/unique/min/max/
  min_length/max_length); enforced in create/update page API (400 + message,
  unique excludes current row); property API accepts 'validation'.
- UI: add-property modal exposes Required/Unique/Min/Max; cell edit shows
  validation errors (red outline + toast) and reverts.
- VERSION + app.main -> 5.1.0; CHANGELOG + ROADMAP updated.
- +9 tests (tests/test_db_advanced.py). 271 passed.

app/templates/_database_table_scripts.html

@@ -1,4 +1,5 @@
 <!-- ═══════════ Database Table Scripts — Vanilla JS ═══════════ -->
+<style>.db-cell-invalid{outline:2px solid #e5484d;outline-offset:-1px;border-radius:4px}</style>
 <script>
 (function() {
     'use strict';
@@ -206,7 +207,31 @@
                 cellEl.innerHTML += '<button class="db-open-btn db-open-page-btn db-hover-only" data-page-id="'+pageId+'" title="Open" style="visibility:hidden;margin-left:auto;flex-shrink:0">↗</button>';
             }
             cellEl.onclick = function(e) { if(!state.editingCell && !e.target.closest('.db-open-btn')) startEditCell(pageId,propId,cellEl); };
-            fetch('/db/pages/'+pageId+'/api',{method:'PUT',headers:{'Content-Type':'application/json','X-CSRF-Token':getCsrf()},body:JSON.stringify({title:cp.title,properties:pv})}).catch(function(e){});
+            fetch('/db/pages/'+pageId+'/api',{method:'PUT',headers:{'Content-Type':'application/json','X-CSRF-Token':getCsrf()},body:JSON.stringify({title:cp.title,properties:pv})})
+              .then(function(r){
+                if(!r.ok){
+                  return r.json().then(function(d){ throw {status:r.status, detail:(d&&d.detail)||('Error '+r.status)}; });
+                }
+                cellEl.classList.remove('db-cell-invalid');
+                return r.json();
+              })
+              .catch(function(err){
+                // Validation failed (400) → show message + revert the cell.
+                const msg = (err && err.detail) ? String(err.detail) : ((err&&err.message)||'Failed to save');
+                if(err && err.status===400){
+                  cellEl.classList.add('db-cell-invalid');
+                  cellEl.title = msg;
+                }
+                if(typeof window.showToast==='function') window.showToast('⚠ '+msg,'error');
+                else alert('⚠ '+msg);
+                // Revert to the stored value.
+                const stored = pv[String(propId)];
+                cellEl.innerHTML = '<span class="db-cell-value">'+escHtml(stored||'\u2014')+'</span>';
+                if (prop.prop_type === 'title') {
+                  cellEl.innerHTML += '<button class="db-open-btn db-open-page-btn db-hover-only" data-page-id="'+pageId+'" title="Open" style="visibility:hidden;margin-left:auto;flex-shrink:0">↗</button>';
+                }
+                cellEl.onclick = function(e) { if(!state.editingCell && !e.target.closest('.db-open-btn')) startEditCell(pageId,propId,cellEl); };
+              });
         }
 
         function cancelEditCell() {
@@ -246,6 +271,14 @@
               '<div class="db-modal-body" style="padding:16px">'+
               '<div class="db-modal-field" style="margin-bottom:12px"><label style="display:block;margin-bottom:4px;font-size:12px;color:var(--text-dim)">Name</label><input class="db-prop-name-inline" type="text" placeholder="Property name" style="width:100%;padding:8px 10px;border:1px solid var(--border);border-radius:6px;font-size:14px;box-sizing:border-box"></div>'+
               '<div class="db-modal-field" style="margin-bottom:12px"><label style="display:block;margin-bottom:4px;font-size:12px;color:var(--text-dim)">Type</label><select class="db-prop-type-inline" style="width:100%;padding:8px 10px;border:1px solid var(--border);border-radius:6px;font-size:14px;box-sizing:border-box"><option value="text">Text</option><option value="number">Number</option><option value="select">Select</option><option value="multi_select">Multi-select</option><option value="date">Date</option><option value="checkbox">Checkbox</option><option value="url">URL</option><option value="email">Email</option><option value="phone">Phone</option></select></div>'+
+              '<div style="display:flex;gap:16px;margin-bottom:12px">'+
+                '<label style="display:flex;align-items:center;gap:6px;font-size:12px;color:var(--text)"><input class="db-prop-required-inline" type="checkbox"> Required</label>'+
+                '<label style="display:flex;align-items:center;gap:6px;font-size:12px;color:var(--text)"><input class="db-prop-unique-inline" type="checkbox"> Unique</label>'+
+              '</div>'+
+              '<div style="display:flex;gap:10px;margin-bottom:12px">'+
+                '<div style="flex:1"><label style="display:block;margin-bottom:4px;font-size:12px;color:var(--text-dim)">Min</label><input class="db-prop-min-inline" type="number" placeholder="—" style="width:100%;padding:7px 10px;border:1px solid var(--border);border-radius:6px;font-size:13px;box-sizing:border-box"></div>'+
+                '<div style="flex:1"><label style="display:block;margin-bottom:4px;font-size:12px;color:var(--text-dim)">Max</label><input class="db-prop-max-inline" type="number" placeholder="—" style="width:100%;padding:7px 10px;border:1px solid var(--border);border-radius:6px;font-size:13px;box-sizing:border-box"></div>'+
+              '</div>'+
               '<div style="display:flex;justify-content:flex-end;gap:8px;margin-top:16px"><button class="db-prop-cancel-inline" style="padding:6px 14px;border-radius:6px;border:1px solid var(--border);background:var(--bg-primary);color:var(--text-secondary);font-size:13px;cursor:pointer">Cancel</button><button class="db-prop-create-inline" style="padding:6px 14px;border-radius:6px;border:none;background:var(--accent,#2383E2);color:white;font-size:13px;cursor:pointer">Create</button></div>'+
               '</div></div>';
             document.body.appendChild(modal);
@@ -261,10 +294,17 @@
                 var name = nameInput.value.trim();
                 if(!name) return;
                 var type = modal.querySelector('.db-prop-type-inline').value;
+                var validation = {};
+                if(modal.querySelector('.db-prop-required-inline').checked) validation.required = true;
+                if(modal.querySelector('.db-prop-unique-inline').checked) validation.unique = true;
+                var min = modal.querySelector('.db-prop-min-inline').value;
+                var max = modal.querySelector('.db-prop-max-inline').value;
+                if(min!=='') validation.min = Number(min);
+                if(max!=='') validation.max = Number(max);
                 try {
                     var r = await fetch('/db/'+collectionId+'/properties/api',{
                         method:'POST',headers:{'Content-Type':'application/json','X-CSRF-Token':getCsrf()},
-                        body:JSON.stringify({name:name,prop_type:type})
+                        body:JSON.stringify({name:name,prop_type:type,validation:validation})
                     });
                     var d = await r.json();
                     if(d.id){ state.properties.push(d); render(); modal.remove(); }