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/user.py

@@ -1,9 +1,8 @@
 import uuid
 from datetime import datetime
 
-from sqlalchemy import Boolean, DateTime, String, func
-from sqlalchemy.dialects.postgresql import JSONB
-from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy import Boolean, DateTime, ForeignKey, String, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.models.base import Base
 
@@ -20,10 +19,15 @@ class User(Base):
     is_active: Mapped[bool] = mapped_column(Boolean, default=True, nullable=False)
     display_name: Mapped[str | None] = mapped_column(String(50))
     theme: Mapped[str | None] = mapped_column(String(20))
-    # Only meaningful when theme == "custom" -- kept even if the user
-    # switches to a preset and back, so switching away from custom is never
-    # destructive. Shape is CustomThemeColors (backend/app/schemas/user.py).
-    custom_theme_colors: Mapped[dict | None] = mapped_column(JSONB)
+    # Only meaningful when theme == "custom" -- which of this user's saved
+    # CustomTheme rows (app/models/custom_theme.py) is currently active.
+    # Cleared explicitly (not via a DB-level ON DELETE) whenever that theme
+    # is deleted -- see custom_theme_service.delete_custom_theme, which also
+    # resets `theme` back to a preset in the same transaction so the two
+    # columns can't go out of sync.
+    active_custom_theme_id: Mapped[uuid.UUID | None] = mapped_column(
+        ForeignKey("custom_themes.id"), nullable=True
+    )
     avatar_filename: Mapped[str | None] = mapped_column(String(64))
     avatar_content_type: Mapped[str | None] = mapped_column(String(50))
     # Manual override for the presence indicator -- when set, this user
@@ -33,3 +37,5 @@ class User(Base):
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now(), nullable=False
     )
+
+    active_custom_theme = relationship("CustomTheme", foreign_keys=[active_custom_theme_id])