feat(m5b): optional YouTube write scope via incremental OAuth

Default login now requests read-only (youtube.readonly); write (unsubscribe,
later playlist export) is an explicit opt-in.

- auth.py: split READ_SCOPES / WRITE_SCOPES; new GET /auth/upgrade (incremental
  consent, prompt=consent); has_write_scope() helper
- /api/me exposes can_write
- youtube/client.py: delete_subscription (50 units, OAuth-only)
- DELETE /api/channels/{id}/subscription, gated on write scope (403 otherwise)
- UI: Settings - Account 'Playlist editing & YouTube export' enable button;
  per-channel 'Unsubscribe on YouTube' (with confirm) shown only when can_write

Browser-facing; develop/test locally until the public HTTPS login lands. Needs a
one-time Console step: add youtube.readonly to the OAuth consent screen scopes.

backend/app/auth.py

@@ -11,9 +11,12 @@ from app.db import get_db
 from app.models import OAuthToken, User
 from app.security import encrypt
 
-# Single YouTube scope that allows reading subscriptions/playlists AND writing
-# (unsubscribe, playlist export). openid/email/profile give us the account identity.
-SCOPES = "openid email profile https://www.googleapis.com/auth/youtube"
+# YouTube's full scope (read + write: unsubscribe, playlist export) and its read-only
+# counterpart. Default login asks for read-only; write is an explicit opt-in via
+# /auth/upgrade (incremental consent), so friends who won't grant write can still browse.
+WRITE_SCOPE = "https://www.googleapis.com/auth/youtube"
+READ_SCOPES = f"openid email profile {WRITE_SCOPE}.readonly"
+WRITE_SCOPES = f"openid email profile {WRITE_SCOPE}"
 
 log = logging.getLogger("subfeed.auth")
 
@@ -25,10 +28,18 @@ oauth.register(
     client_id=settings.google_client_id,
     client_secret=settings.google_client_secret,
     server_metadata_url="https://accounts.google.com/.well-known/openid-configuration",
-    client_kwargs={"scope": SCOPES},
+    client_kwargs={"scope": READ_SCOPES},
 )
 
 
+def has_write_scope(user: User) -> bool:
+    """Whether the user's stored grant includes YouTube write (unsubscribe / export)."""
+    tok = user.token
+    if tok is None or not tok.scopes:
+        return False
+    return WRITE_SCOPE in tok.scopes.split()
+
+
 @router.get("/login")
 async def login(request: Request):
     # access_type=offline ensures a refresh_token on first authorization. We avoid
@@ -40,6 +51,7 @@ async def login(request: Request):
         access_type="offline",
         prompt="select_account",
         include_granted_scopes="true",
+        scope=READ_SCOPES,
     )
 
 
@@ -80,7 +92,7 @@ async def callback(request: Request, db: Session = Depends(get_db)):
     tok.expiry = (
         datetime.fromtimestamp(expires_at, tz=timezone.utc) if expires_at else None
     )
-    tok.scopes = token.get("scope") or SCOPES
+    tok.scopes = token.get("scope") or READ_SCOPES
     db.add(tok)
     db.commit()
 
@@ -106,6 +118,20 @@ def current_user(request: Request, db: Session = Depends(get_db)) -> User:
     return user
 
 
+@router.get("/upgrade")
+async def upgrade(request: Request, user: User = Depends(current_user)):
+    """Incremental consent: re-authorize with the full YouTube (write) scope. The shared
+    callback stores the new scope set, so `can_write` flips on once Google grants it."""
+    return await oauth.google.authorize_redirect(
+        request,
+        settings.oauth_redirect_url,
+        access_type="offline",
+        prompt="consent",
+        include_granted_scopes="true",
+        scope=WRITE_SCOPES,
+    )
+
+
 @router.get("/me")
 async def me(user: User = Depends(current_user)) -> dict:
     return {

backend/app/routes/channels.py

@@ -5,9 +5,10 @@ from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy import and_, func, select
 from sqlalchemy.orm import Session
 
-from app.auth import current_user
+from app.auth import current_user, has_write_scope
 from app.db import get_db
 from app.models import Channel, ChannelTag, Subscription, Tag, User, Video
+from app.youtube.client import YouTubeClient, YouTubeError
 
 router = APIRouter(prefix="/api/channels", tags=["channels"])
 
@@ -101,6 +102,35 @@ def update_channel(
     }
 
 
+@router.delete("/{channel_id}/subscription")
+def unsubscribe(
+    channel_id: str,
+    user: User = Depends(current_user),
+    db: Session = Depends(get_db),
+) -> dict:
+    """Unsubscribe from this channel on YouTube and drop the local subscription. Gated
+    behind the optional write scope; read-only users get a 403 and should Hide instead."""
+    if not has_write_scope(user):
+        raise HTTPException(
+            status_code=403,
+            detail="Enable playlist editing in Settings to unsubscribe on YouTube.",
+        )
+    sub = _user_subscription(db, user, channel_id)
+    if not sub.yt_subscription_id:
+        raise HTTPException(
+            status_code=400,
+            detail="No YouTube subscription id on record — sync subscriptions first.",
+        )
+    try:
+        with YouTubeClient(db, user) as yt:
+            yt.delete_subscription(sub.yt_subscription_id)
+    except YouTubeError as exc:
+        raise HTTPException(status_code=502, detail=f"YouTube unsubscribe failed: {exc}")
+    db.delete(sub)
+    db.commit()
+    return {"unsubscribed": channel_id}
+
+
 @router.post("/{channel_id}/tags")
 def attach_tag(
     channel_id: str,

backend/app/routes/me.py

@@ -1,7 +1,7 @@
 from fastapi import APIRouter, Depends
 from sqlalchemy.orm import Session
 
-from app.auth import current_user
+from app.auth import current_user, has_write_scope
 from app.db import get_db
 from app.models import User
 
@@ -16,6 +16,7 @@ def get_me(user: User = Depends(current_user)) -> dict:
         "display_name": user.display_name,
         "avatar_url": user.avatar_url,
         "role": user.role,
+        "can_write": has_write_scope(user),
         "preferences": user.preferences or {},
     }
 

backend/app/youtube/client.py

@@ -151,6 +151,25 @@ class YouTubeClient:
             params["pageToken"] = page_token
         return self._get("playlistItems", params)
 
+    def delete_subscription(self, subscription_id: str) -> None:
+        """Unsubscribe on YouTube. Requires the write scope and the user's OAuth token
+        (never the public API key). subscriptions.delete costs 50 quota units."""
+        resp = self._http.delete(
+            f"{API_BASE}/subscriptions",
+            params={"id": subscription_id},
+            headers={"Authorization": f"Bearer {self._access_token()}"},
+        )
+        quota.record_usage(self.db, 50)
+        if resp.status_code not in (200, 204):
+            log.warning(
+                "YouTube subscriptions.delete -> %s: %s",
+                resp.status_code,
+                resp.text[:200],
+            )
+            raise YouTubeError(
+                f"DELETE subscriptions -> {resp.status_code}: {resp.text[:300]}"
+            )
+
     def get_videos(self, video_ids: list[str]) -> list[dict]:
         items: list[dict] = []
         for batch in _chunks(video_ids, 50):