fix(auth): address PR #758 auto-review (id-token verify, nonce, CI key)

Blocking:
- AS proxy callback now calls verify_id_token before caching the proxy
  code so a tampered IdP response can't smuggle identity claims.

Important:
- Browser OAuth flow generates and verifies an OIDC nonce; new alembic
  migration 006 adds the nonce column to oauth_sessions.
- _origin_matches_self logs a warning when CSRF check is bypassed.
- oauth_tools.py uses get_shared_storage instead of fresh handles.

Nits:
- New token_utils.get_oidc_discovery shares the 5-minute cache with
  verify_id_token; oauth_login (integrated) and _revoke_refresh_token_at_idp
  now use it instead of issuing fresh discovery fetches.
- Drop typing.Optional from oauth_tools.py in favour of X | None.

CI:
- test.yml generates an ephemeral Fernet TOKEN_ENCRYPTION_KEY per run
  with openssl, removing the dependency on a missing repo secret.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/alembic/versions/20260502_1600_006_add_nonce_to_oauth_sessions.py

@@ -0,0 +1,29 @@
+"""Add nonce column to oauth_sessions for OIDC ID-token binding.
+
+PR #758 finding 2: the browser OAuth flow generated PKCE + state but no
+``nonce``. Without a nonce, an attacker who obtains a valid ID token for
+another user (e.g. from a parallel auth request) could replay it inside
+this flow because the token isn't cryptographically tied to the
+authorization request. The nonce is generated in ``oauth_login``,
+forwarded to the IdP in the auth URL, and verified on the way back.
+
+Revision ID: 006
+Revises: 005
+Create Date: 2026-05-02 16:00:00.000000
+"""
+
+from alembic import op
+
+revision = "006"
+down_revision = "005"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.execute("ALTER TABLE oauth_sessions ADD COLUMN nonce TEXT")
+
+
+def downgrade() -> None:
+    # SQLite < 3.35 cannot DROP COLUMN; leave the column on downgrade.
+    pass