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_content.html

@@ -581,4 +581,43 @@
         ></span>
         <span class="statusbar-right" x-text="blocks.length+' blocks'"></span>
     </div>
+
+    <!-- ═══════════ Database template picker (v5.3.0) ═══════════ -->
+    <div class="fd-dbtpl-overlay" id="fd-dbtpl" x-show="dbTplOpen"
+         style="display:none;position:fixed;inset:0;background:rgba(0,0,0,.55);z-index:2100;
+                align-items:flex-start;justify-content:center;padding-top:10vh;opacity:0;transition:opacity .15s ease;"
+         @click.self="closeDbTemplatePicker()">
+      <div class="fd-dbtpl" style="width:620px;max-width:94vw;max-height:76vh;display:flex;flex-direction:column;
+           background:var(--bg-secondary,#232323);border:1px solid var(--border,rgba(255,255,255,.08));border-radius:12px;
+           box-shadow:0 24px 70px rgba(0,0,0,.6);overflow:hidden;">
+        <div style="display:flex;align-items:center;justify-content:space-between;padding:16px 20px;border-bottom:1px solid var(--border);">
+          <h3 style="margin:0;font-size:16px;color:var(--text,#fff);">Create a database</h3>
+          <button class="flowdeck-modal-close" @click="closeDbTemplatePicker()">&times;</button>
+        </div>
+        <div style="padding:12px 20px;overflow-y:auto;">
+          <div x-show="dbTplLoading" style="color:var(--text-dim,#999);font-size:13px;padding:20px 0;text-align:center;">Loading templates…</div>
+          <div x-show="!dbTplLoading">
+            <!-- Blank -->
+            <div class="fd-dbtpl-card" @click="createDbFromTemplate(null)"
+                 style="display:flex;align-items:center;gap:12px;padding:12px 14px;border:1px solid var(--border);border-radius:10px;margin-bottom:10px;cursor:pointer;">
+              <span style="font-size:22px;">📋</span>
+              <div><div style="font-weight:600;color:var(--text,#fff);font-size:14px;">Blank database</div>
+                <div style="font-size:12px;color:var(--text-dim,#999);">Start from an empty database</div></div>
+            </div>
+            <template x-for="t in dbTemplates" :key="t.id">
+              <div class="fd-dbtpl-card" @click="createDbFromTemplate(t.name)"
+                   style="display:flex;align-items:center;gap:12px;padding:12px 14px;border:1px solid var(--border);border-radius:10px;margin-bottom:10px;cursor:pointer;">
+                <span style="font-size:22px;" x-text="t.icon||'📋'"></span>
+                <div style="flex:1;"><div style="font-weight:600;color:var(--text,#fff);font-size:14px;" x-text="t.name"></div>
+                  <div style="font-size:12px;color:var(--text-dim,#999);" x-text="t.description||''"></div></div>
+              </div>
+            </template>
+            <div x-show="!dbTplLoading && !dbTemplates.length" style="color:var(--text-dim,#999);font-size:13px;padding:16px 0;text-align:center;">
+              No templates available
+            </div>
+          </div>
+        </div>
+      </div>
+    </div>
+
 </div>