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/migrations.py

@@ -169,4 +169,27 @@ def _migration_fts5(conn: sqlite3.Connection) -> None:
         SELECT id, COALESCE(title, ''), COALESCE(content, '') FROM pages
         WHERE deleted_at IS NULL
         """
-    )
+    )
+
+
+@register(4, "database templates (icon) + property validation")
+def _migration_db_templates_validation(conn: sqlite3.Connection) -> None:
+    """v5.3.0: database templates get an icon, properties a validation config,
+    and the built-in database templates are seeded (idempotently)."""
+    _cols = {r[1] for r in conn.execute("PRAGMA table_info(database_templates)").fetchall()}
+    if "icon" not in _cols:
+        conn.execute("ALTER TABLE database_templates ADD COLUMN icon TEXT NOT NULL DEFAULT '📋'")
+
+    _pcols = {r[1] for r in conn.execute("PRAGMA table_info(collection_properties)").fetchall()}
+    if "validation_json" not in _pcols:
+        conn.execute("ALTER TABLE collection_properties ADD COLUMN validation_json TEXT NOT NULL DEFAULT '{}'")
+
+    # Seed built-in templates (idempotent: only missing names are inserted).
+    from app.services.db_templates import SEED_TEMPLATES
+    for tpl in SEED_TEMPLATES:
+        conn.execute(
+            """INSERT OR IGNORE INTO database_templates (name, icon, description, schema_json)
+               VALUES (?, ?, ?, ?)""",
+            (tpl["name"], tpl.get("icon", "📋"), tpl.get("description", ""),
+             __import__("json").dumps(tpl.get("schema", []))),
+        )