feat(account): GDPR self-service account + data deletion (5d)

Add DELETE /api/me/account: permanently erases the signed-in account and all its
personal data — OAuth tokens, subscriptions, tags, video states, playlists,
notifications all cascade on the users row (FK ON DELETE CASCADE); quota-audit
events are kept but anonymised (SET NULL); the email's invite/whitelist row is
removed too. Guards: the shared demo account can't be deleted, and the last
remaining admin can't delete itself. Frontend: a Danger zone in Settings -> Account
(non-demo) with a danger-confirm; on success the session clears and the app lands
on the welcome page. EN/HU/DE. Verified via curl: self-delete + session clear +
demo 403.

backend/app/routes/me.py

@@ -1,5 +1,5 @@
 from fastapi import APIRouter, Depends, HTTPException, Request
-from sqlalchemy import func, select
+from sqlalchemy import delete, func, select
 from sqlalchemy.orm import Session
 
 from app.auth import current_user, has_read_scope, has_write_scope, is_allowed
@@ -85,6 +85,43 @@ def get_me(
     }
 
 
+@router.delete("/account")
+def delete_my_account(
+    request: Request,
+    user: User = Depends(current_user),
+    db: Session = Depends(get_db),
+) -> dict:
+    """GDPR self-service erasure: permanently delete the signed-in account and ALL its personal
+    data — OAuth tokens, subscriptions, tags, watch/save/hide states, playlists, notifications —
+    which all cascade on the users row. Quota-audit events are kept but anonymised (FK SET NULL).
+    The shared demo account can't be deleted, and the last remaining admin can't delete itself
+    (that would lock everyone out of the admin surfaces)."""
+    if user.is_demo:
+        raise HTTPException(status_code=403, detail="The shared demo account can't be deleted.")
+    if user.role == "admin":
+        admins = db.scalar(select(func.count()).select_from(User).where(User.role == "admin")) or 0
+        if admins <= 1:
+            raise HTTPException(
+                status_code=400,
+                detail="You're the only admin — promote another admin before deleting your account.",
+            )
+    email = user.email.lower()
+    user_id = user.id
+    # Raw delete so Postgres applies the ON DELETE CASCADE / SET NULL on every dependent table.
+    db.execute(delete(User).where(User.id == user_id))
+    # Erase the access-request/whitelist row for this email too (full erasure; they'd re-request).
+    db.execute(delete(Invite).where(Invite.email == email))
+    db.commit()
+    # Drop this account from the browser session; switch to another signed-in account if any.
+    remaining = [a for a in (request.session.get("account_ids") or []) if a != user_id]
+    if remaining:
+        request.session["account_ids"] = remaining
+        request.session["user_id"] = remaining[-1]
+    else:
+        request.session.clear()
+    return {"deleted": True}
+
+
 @router.put("/preferences")
 def update_preferences(
     preferences: dict,