fix(security): patch cryptography CVEs, upgrade pip at build, harden /auth/upgrade

- requirements: cryptography >=46.0.7 (was pinned <46, which excluded the fix for
  the CVEs pip-audit flagged in our Fernet/crypto library). pip-audit now clean.
- Dockerfile: upgrade pip before installing deps (patches installer-level CVEs).
- auth: /auth/upgrade now defaults to the least-privileged read scope; only an
  explicit access=write requests the write scope.

backend/app/auth.py

@@ -213,13 +213,14 @@ def current_user(request: Request, db: Session = Depends(get_db)) -> User:
 
 @router.get("/upgrade")
 async def upgrade(
-    request: Request, access: str = "write", user: User = Depends(current_user)
+    request: Request, access: str = "read", user: User = Depends(current_user)
 ):
     """Incremental consent for the onboarding wizard. `access=read` grants YouTube
     read-only (enough to build the feed); `access=write` additionally grants management
     (unsubscribe). The shared callback stores whatever Google returns, so `can_read` /
-    `can_write` flip on once granted."""
-    scope = READ_SCOPES if access == "read" else WRITE_SCOPES
+    `can_write` flip on once granted. Anything other than an explicit `write` defaults to
+    the least-privileged read scope."""
+    scope = WRITE_SCOPES if access == "write" else READ_SCOPES
     return await oauth.google.authorize_redirect(
         request,
         settings.oauth_redirect_url,