fix: Consolidate OAuth callbacks and implement PKCE for all flows

This PR fixes multiple OAuth-related issues:

## Unified OAuth Callback
- Consolidated `/oauth/callback-nextcloud` and `/oauth/login-callback` into single `/oauth/callback` endpoint
- Flow type determined by session lookup via state parameter (no query params in redirect_uri)
- Fixes redirect_uri validation issues with IdPs requiring exact match
- Legacy endpoints kept as aliases for backwards compatibility

## PKCE Implementation
- Implemented PKCE (RFC 7636) for Flow 2 (resource provisioning)
  - Generate code_verifier and code_challenge
  - Store code_verifier in session storage
  - Retrieve and use in token exchange
- Fixed PKCE for browser login (integrated mode)
  - Previously only worked for external IdP (Keycloak)
  - Now works for both Nextcloud OIDC and external IdP

## Login Elicitation Fixes (ADR-006)
- Fixed elicitation URL to route through MCP server endpoint
  - Changed from direct Nextcloud URL to `/oauth/authorize-nextcloud`
  - Ensures PKCE is properly handled by server
- Fixed login detection after OAuth flow completes
  - Look up refresh token by state parameter instead of user_id
  - Works even when Flow 1 token not present
- Added `get_refresh_token_by_provisioning_client_id()` method

## Session Authentication
- Fixed `/user/page` redirect loop
  - Shared oauth_context with mounted browser_app
  - SessionAuthBackend can now validate sessions correctly

## Tests
- Added comprehensive login elicitation test suite
- Updated scope authorization test expectations
- All 43 OAuth tests passing

## Files Changed
- `app.py`: Shared oauth_context, unified callback route
- `oauth_routes.py`: Unified callback, PKCE for Flow 2
- `browser_oauth_routes.py`: PKCE for integrated mode
- `oauth_tools.py`: Fixed elicitation URL generation
- `refresh_token_storage.py`: Added lookup by provisioning_client_id
- `test_login_elicitation.py`: New test suite

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/server/oauth/test_scope_authorization.py

@@ -412,7 +412,7 @@ async def test_jwt_with_no_custom_scopes_returns_zero_tools(
 
     tool_names = [tool.name for tool in result.tools]
     logger.info(
-        f"JWT token with no custom scopes sees {len(tool_names)} tools (should be 3 OAuth tools)"
+        f"JWT token with no custom scopes sees {len(tool_names)} tools (should be 4 OAuth tools)"
     )
 
     # Only OAuth provisioning tools should be visible (they require 'openid' scope)
@@ -420,6 +420,7 @@ async def test_jwt_with_no_custom_scopes_returns_zero_tools(
         "provision_nextcloud_access",
         "revoke_nextcloud_access",
         "check_provisioning_status",
+        "check_logged_in",  # Login elicitation tool (ADR-006)
     ]
 
     assert set(tool_names) == set(expected_oauth_tools), (