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/_page_editor_scripts.html

@@ -10,7 +10,8 @@
       {id:'callout',name:'Callout',icon:'lightbulb'},{id:'table_of_contents',name:'Table of contents',icon:'☰'},{id:'math',name:'Math equation',icon:'∑'},
       {id:'columns',name:'Column list',icon:'▮'},{id:'divider',name:'Divider',icon:'—'},{id:'code',name:'Code block',icon:'<>'},
       {id:'table',name:'Table',icon:'⊞'},
-    ]},{name:'MEDIA',items:[{id:'image',name:'Image',icon:'image'}]}];
+    ]},{name:'MEDIA',items:[{id:'image',name:'Image',icon:'image'}]},
+    {name:'DATA',items:[{id:'database',name:'Database / Inline database',icon:'📊'}]}];
 
     let _bid=0;function genId(){return 'b'+(++_bid)+'_'+Date.now().toString(36);}
     function cp(e){const s=window.getSelection();if(!s.rangeCount)return 0;const r=s.getRangeAt(0).cloneRange();r.selectNodeContents(e);r.setEnd(s.getRangeAt(0).endContainer,s.getRangeAt(0).endOffset);return r.toString().length;}
@@ -871,26 +872,48 @@
         },
         deletePage(){this.moreOpen=false;if(!confirm('Move to Trash?'))return;const csrf=document.cookie.match(/csrf_token=([^;]+)/);fetch(`/board/api/pages/${this.pid}/trash`,{method:'POST',headers:{'X-CSRF-Token':csrf?csrf[1]:''}}).then(()=>{window.location.href='/';}).catch(()=>{window.showToast('Failed','error');});},
 
-        // ── v4.6.0: Get Started actions ──
+        // ── v4.6.0 / v5.3.0: Get Started actions — Database (template picker) ──
+        dbTplOpen:false, dbTplLoading:false, dbTemplates:[], dbTplTargetIdx:-1,
         async createDatabase(){
+          this.openDbTemplatePicker(-1);
+        },
+        async openDbTemplatePicker(targetIdx){
+          this.gsMoreOpen=false;
+          this.dbTplOpen=true; this.dbTplTargetIdx=targetIdx; this.dbTplLoading=true;
+          var self=this;
+          try{
+            const r=await fetch('/workspace/templates/database');
+            const d=await r.json();
+            self.dbTemplates=d.templates||[];
+          }catch(e){ self.dbTemplates=[]; }
+          self.dbTplLoading=false;
+          self.$nextTick(()=>{ const f=document.getElementById('fd-dbtpl'); if(f) setTimeout(()=>{f.style.opacity='1';},10); });
+        },
+        closeDbTemplatePicker(){ this.dbTplOpen=false; },
+        async createDbFromTemplate(tplName){
+          const self=this;
+          var name = tplName ? (tplName+' — '+new Date().toLocaleDateString('fr-FR')) : 'Database';
           const csrf=document.cookie.match(/csrf_token=([^;]+)/);
           try{
-            const name='Database';
-            // Create collection via API
-            const r=await fetch('/db/api',{
+            const body={name:name, parent_page_id:this.pid};
+            if(tplName) body.template=tplName;
+            const r=await fetch('/db/inline/api',{
               method:'POST',headers:{'Content-Type':'application/json','X-CSRF-Token':csrf?csrf[1]:''},
-              body:JSON.stringify({name:name})
+              body:JSON.stringify(body)
             });
             const d=await r.json();
             if(d.id){
-              // Add inline database embed block to the page
-              this.blocks.push(this.mkB('embed','',{embed_type:'collection',collection_id:d.id}));
-              this.render(); this.dirty=true; this.autoSave();
-              this.showToast('Database created');
+              const nb=this.mkB('embed','',{embed_type:'collection',collection_id:d.id});
+              this.sync();
+              const idx=this.dbTplTargetIdx>=0?this.dbTplTargetIdx:(this.blocks.length);
+              this.blocks.splice(idx,0,nb);
+              this.dbTplOpen=false;
+              this.dirty=true; this.autoSave(); this.render();
+              this.showToast(tplName?('Database created from template — '+tplName):'Database created');
             }else{
-              this.showToast('Failed to create database');
+              this.showToast(d.detail||'Failed to create database');
             }
-          }catch(e){this.showToast('Error: '+e.message);}
+          }catch(e){ this.showToast('Error: '+e.message); }
         },
         async createForm(){
           const name=prompt('Form name:','New Form');
@@ -979,6 +1002,7 @@
         },
         applySlash(id){
           const idx=SM._i;if(idx<0||idx>=this.blocks.length)return;
+          if(id==='database'){ this.openDbTemplatePicker(idx); return; }
           const block=this.blocks[idx];this.sync();block.content='';
           if(id==='divider'){block.type='divider';this.addAfter(idx,'paragraph');}
           else{block.type=id;if(id==='toggle'){block.expanded=true;block.children=[];}if(id==='to_do')block.checked=false;if(id==='columns')block.children=[{id:genId(),type:'paragraph',content:''}];if(id==='code')block.language='Plain Text';if(id==='table'){block.rows=_tblNew(3,3);block.content='';}}