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/models/__init__.py

@@ -1,6 +1,7 @@
 from app.models.admin_audit_log import AdminAuditLog
 from app.models.api_token import ApiToken
 from app.models.base import Base
+from app.models.custom_theme import CustomTheme
 from app.models.event_subscription import EventSubscription
 from app.models.invite import InviteStatus
 from app.models.membership import RoomMembership, RoomRole
@@ -37,4 +38,5 @@ __all__ = [
     "ApiToken",
     "WebhookIncoming",
     "EventSubscription",
+    "CustomTheme",
 ]