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/routers/workspace.py

@@ -231,8 +231,9 @@ async def create_db_template(request: Request):
     cur = None
     with get_conn() as conn:
         cur = conn.execute(
-            "INSERT INTO database_templates (name, description, schema_json) VALUES (?,?,?)",
-            (body.get("name", "Template"), body.get("description", ""), json.dumps(body.get("schema", []))),
+            "INSERT INTO database_templates (name, description, icon, schema_json) VALUES (?,?,?,?)",
+            (body.get("name", "Template"), body.get("description", ""),
+             body.get("icon", "📋"), json.dumps(body.get("schema", []))),
         )
         conn.commit()
     return {"id": cur.lastrowid, "status": "created"}
@@ -240,22 +241,16 @@ async def create_db_template(request: Request):
 
 @router.post("/templates/database/{tid}/apply")
 async def apply_db_template(request: Request, tid: int):
+    from app.services.db_templates import create_from_template
     body = await request.json() if request.headers.get("content-type") else {}
     name = body.get("name", "New Database")
     with get_conn() as conn:
         tmpl = conn.execute("SELECT * FROM database_templates WHERE id=?", (tid,)).fetchone()
         if not tmpl:
             raise HTTPException(404, "Template not found")
-        cur = conn.execute(
-            "INSERT INTO collections (name, description, icon, schema_json) VALUES (?,?,?,?)",
-            (name, tmpl["description"], "📋", tmpl["schema_json"]),
-        )
-        conn.execute(
-            "INSERT INTO collection_views (collection_id, name, view_type, config_json) VALUES (?,?,?,?)",
-            (cur.lastrowid, "Default View", "table", "{}"),
-        )
+        collection_id = create_from_template(conn, name, dict(tmpl))
         conn.commit()
-    return {"collection_id": cur.lastrowid, "name": name, "status": "created"}
+    return {"collection_id": collection_id, "name": name, "status": "created"}
 
 
 @router.get("/collections/{collection_id}/templates/page")