refactor(config)!: rename OAUTH_SINGLE_AUDIENCE to LOGIN_FLOW, gate on ENABLE_LOGIN_FLOW

The AuthMode.OAUTH_SINGLE_AUDIENCE enum was a vestige of ADR-021's
original design where it co-existed with OAUTH_TOKEN_EXCHANGE. The
un-augmented OAuth bearer pass-through it represented relied on
Nextcloud-side patches to user_oidc (Bearer token validation on
non-OCS endpoints) that were never merged upstream (see
docs/authentication.md, docs/login-flow-v2.md). The working path —
mcp-login-flow profile — sets ENABLE_LOGIN_FLOW=true on top of this
mode so Login Flow v2 acquires per-user Nextcloud app passwords via
a browser flow. With OAUTH_TOKEN_EXCHANGE removed in 57303135, the
_AUDIENCE suffix in the Python name no longer disambiguates anything,
and the enum value diverged from the env-var spelling. ADR-022 (now
accepted) called for this rename as step 1 of consolidation.

- nextcloud_mcp_server/config_validators.py: rename enum to LOGIN_FLOW
  with value "login_flow". The mode_map key is now "login_flow"; the
  MODE_REQUIREMENTS entry requires `enable_login_flow=True`. Added a
  validation gate so MCP_DEPLOYMENT_MODE=login_flow without
  ENABLE_LOGIN_FLOW=true errors with a clear message pointing at
  ADR-022. Default auto-detection fallback returns LOGIN_FLOW.
- nextcloud_mcp_server/app.py: renamed three identifier references and
  switched the "Configuring MCP server for OAuth mode" log line to
  the uniform `mode.value` shape used by the other modes.
- nextcloud_mcp_server/api/management.py: renamed identifier in the
  /api/v1/status mapping. The user-visible "auth_mode": "oauth" string
  is preserved — that's a stable Astrolabe contract.
- nextcloud_mcp_server/config.py: updated Settings docstring.
- tests/unit/test_config_validators.py: renamed class
  TestOAuthSingleAudienceValidation → TestLoginFlowValidation,
  individual test methods, env-var strings; added enable_login_flow=True
  to fixtures expecting success; added a new test
  (test_login_flow_requires_enable_login_flow_flag) that exercises the
  validation gate.
- tests/unit/test_management_status_endpoint.py: renamed identifier.

BREAKING CHANGE: MCP_DEPLOYMENT_MODE=oauth_single_audience is no longer
accepted. Set MCP_DEPLOYMENT_MODE=login_flow (and keep
ENABLE_LOGIN_FLOW=true) for the same deployment. The un-augmented
OAuth path is no longer supported; if you previously ran the broken
path, you can either configure Login Flow v2 (recommended) or switch
to multi_user_basic / single_user_basic.

Dead-code pruning of `oauth_enabled and not enable_login_flow`
branches in app.py (lifespan, background sync) is deferred to a
separate follow-up PR per the consolidation plan in ADR-022.

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

nextcloud_mcp_server/config.py

@@ -408,9 +408,10 @@ def get_document_processor_config() -> dict[str, Any]:
 class Settings:
     """Application settings from environment variables."""
 
-    # Deployment mode (ADR-021: explicit mode selection)
+    # Deployment mode (ADR-021: explicit mode selection; updated by ADR-022)
     # Optional: If not set, mode is auto-detected from other settings
-    # Valid values: single_user_basic, multi_user_basic, oauth_single_audience
+    # Valid values: single_user_basic, multi_user_basic, login_flow
+    # (ADR-022: `oauth_single_audience` was renamed to `login_flow`.)
     deployment_mode: str | None = None
 
     # OAuth/OIDC settings