Support multiple named, saved custom themes per user

Replaces the single custom_theme_colors blob (one palette per user) with
a proper CustomTheme table -- users can now save, name, and switch
between as many custom palettes as they like, not just one.

Data model: users.active_custom_theme_id references whichever saved
CustomTheme (if any) is currently active; theme='custom' + that id
together determine what's rendered. The migration data-migrates any
already-saved single palette into a named CustomTheme row on upgrade,
and best-effort backfills the active one back into the old column shape
on downgrade.

New endpoints under /api/custom-themes: list, create, rename/recolor,
delete (falls back the user to a preset if the deleted theme was
active, so the two theme columns can never disagree), and activate.
UserRead.active_custom_theme is only populated when theme == 'custom'
even though the DB deliberately keeps the id set while a preset is
active, so switching to a preset and back doesn't lose the saved
palette.

ProfileModal now lists saved themes as swatches (click to activate,
pencil to edit -- active or not, trash to delete with a confirm), plus
a "+ New" button that creates, activates, and opens the editor for a
fresh theme immediately.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/main.py

@@ -11,7 +11,19 @@ from redis.asyncio import Redis
 from starlette.middleware.sessions import SessionMiddleware
 
 from app.config import settings
-from app.routers import admin, auth, bots, health, push, rooms, signup, uploads, users, webhooks
+from app.routers import (
+    admin,
+    auth,
+    bots,
+    custom_themes,
+    health,
+    push,
+    rooms,
+    signup,
+    uploads,
+    users,
+    webhooks,
+)
 from app.ws.broadcaster import Broadcaster
 from app.ws.chat import router as ws_router
 from app.ws.connection_manager import ConnectionManager
@@ -78,6 +90,7 @@ def create_app() -> FastAPI:
     app.include_router(rooms.router)
     app.include_router(users.router)
     app.include_router(push.router)
+    app.include_router(custom_themes.router)
     app.include_router(uploads.router)
     app.include_router(admin.router)
     app.include_router(bots.router)