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/services/auth_service.py

@@ -1,6 +1,7 @@
 from sqlalchemy import or_, select
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import selectinload
 
 from app.models import User
 from app.schemas.user import UserCreate
@@ -40,9 +41,9 @@ async def authenticate_user(
     db: AsyncSession, username_or_email: str, password: str
 ) -> User:
     result = await db.execute(
-        select(User).where(
-            or_(User.username == username_or_email, User.email == username_or_email)
-        )
+        select(User)
+        .where(or_(User.username == username_or_email, User.email == username_or_email))
+        .options(selectinload(User.active_custom_theme))
     )
     user = result.scalar_one_or_none()
     if user is None or not verify_password(password, user.password_hash):

backend/app/services/custom_theme_service.py

@@ -0,0 +1,107 @@
+import uuid
+
+from sqlalchemy import func, select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models import CustomTheme, User
+from app.schemas.custom_theme import CustomThemeColors
+
+# Sane cap, not a hard product requirement -- keeps the swatch list from
+# growing unbounded and matches this app's generally modest per-user scale
+# (self-hosted, small groups) elsewhere.
+MAX_CUSTOM_THEMES_PER_USER = 20
+
+
+class CustomThemeNotFoundError(Exception):
+    pass
+
+
+class CustomThemeLimitReachedError(Exception):
+    pass
+
+
+async def list_custom_themes(db: AsyncSession, user_id: uuid.UUID) -> list[CustomTheme]:
+    result = await db.execute(
+        select(CustomTheme)
+        .where(CustomTheme.user_id == user_id)
+        .order_by(CustomTheme.created_at)
+    )
+    return list(result.scalars().all())
+
+
+async def create_custom_theme(
+    db: AsyncSession, user_id: uuid.UUID, name: str, colors: CustomThemeColors
+) -> CustomTheme:
+    count = await db.scalar(
+        select(func.count()).select_from(CustomTheme).where(CustomTheme.user_id == user_id)
+    )
+    if count >= MAX_CUSTOM_THEMES_PER_USER:
+        raise CustomThemeLimitReachedError()
+
+    theme = CustomTheme(user_id=user_id, name=name, colors=colors.model_dump())
+    db.add(theme)
+    await db.commit()
+    await db.refresh(theme)
+    return theme
+
+
+async def _get_owned_theme(db: AsyncSession, user_id: uuid.UUID, theme_id: uuid.UUID) -> CustomTheme:
+    theme = await db.get(CustomTheme, theme_id)
+    if theme is None or theme.user_id != user_id:
+        raise CustomThemeNotFoundError()
+    return theme
+
+
+async def update_custom_theme(
+    db: AsyncSession,
+    user_id: uuid.UUID,
+    theme_id: uuid.UUID,
+    name: str | None,
+    colors: CustomThemeColors | None,
+) -> CustomTheme:
+    theme = await _get_owned_theme(db, user_id, theme_id)
+    if name is not None:
+        theme.name = name
+    if colors is not None:
+        theme.colors = colors.model_dump()
+    await db.commit()
+    await db.refresh(theme)
+    return theme
+
+
+async def delete_custom_theme(db: AsyncSession, user_id: uuid.UUID, theme_id: uuid.UUID) -> None:
+    theme = await _get_owned_theme(db, user_id, theme_id)
+    user = await db.get(User, user_id)
+    # A deleted-but-still-active theme would otherwise leave theme='custom'
+    # pointing at nothing -- fall back to a preset so the two columns can
+    # never disagree about what's actually being displayed.
+    if user.active_custom_theme_id == theme.id:
+        user.active_custom_theme_id = None
+        # Keep the relationship attribute in sync too, not just the raw FK
+        # column -- if `user` is already identity-mapped in this session
+        # (e.g. a later db.get() in the same request/connection returns the
+        # same Python object rather than re-querying), only the FK column
+        # being updated would leave .active_custom_theme still pointing at
+        # the object we're about to delete below.
+        user.active_custom_theme = None
+        user.theme = "dark"
+        # Flush the FK-clearing UPDATE before the DELETE below -- setting
+        # the raw *_id column directly (not the relationship attribute)
+        # doesn't register with SQLAlchemy's automatic flush-order
+        # dependency detection, so without this the DELETE can be sent
+        # first and trip the foreign key constraint.
+        await db.flush()
+    await db.delete(theme)
+    await db.commit()
+
+
+async def activate_custom_theme(db: AsyncSession, user_id: uuid.UUID, theme_id: uuid.UUID) -> None:
+    theme = await _get_owned_theme(db, user_id, theme_id)
+    user = await db.get(User, user_id)
+    user.theme = "custom"
+    user.active_custom_theme_id = theme_id
+    # See delete_custom_theme's comment -- keep the relationship attribute
+    # in sync too, in case `user` is already identity-mapped elsewhere in
+    # this session.
+    user.active_custom_theme = theme
+    await db.commit()

backend/app/services/password_service.py

@@ -3,6 +3,7 @@ from datetime import datetime, timezone
 
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import selectinload
 
 from app.models import PasswordReset, User
 from app.security import hash_password, hash_token, verify_password
@@ -70,7 +71,9 @@ async def validate_reset_token(db: AsyncSession, token: str) -> None:
 
 async def complete_password_reset(db: AsyncSession, token: str, new_password: str) -> User:
     reset = await _get_valid_reset(db, token)
-    user = await db.get(User, reset.user_id)
+    user = await db.get(
+        User, reset.user_id, options=[selectinload(User.active_custom_theme)]
+    )
     user.password_hash = hash_password(new_password)
     reset.used = True
     await db.commit()