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>

keycloak/realm-export.json

@@ -751,6 +751,40 @@
         "display.on.consent.screen": "true",
         "consent.screen.text": "Create, update, and delete tasks"
       }
+    },
+    {
+      "name": "default-audience",
+      "protocol": "openid-connect",
+      "attributes": {
+        "include.in.token.scope": "false",
+        "display.on.consent.screen": "false",
+        "gui.order": "",
+        "consent.screen.text": ""
+      },
+      "protocolMappers": [
+        {
+          "name": "mcp-server-audience",
+          "protocol": "openid-connect",
+          "protocolMapper": "oidc-audience-mapper",
+          "consentRequired": false,
+          "config": {
+            "included.client.audience": "nextcloud-mcp-server",
+            "access.token.claim": "true",
+            "id.token.claim": "false"
+          }
+        },
+        {
+          "name": "mcp-url-audience",
+          "protocol": "openid-connect",
+          "protocolMapper": "oidc-audience-mapper",
+          "consentRequired": false,
+          "config": {
+            "included.custom.audience": "http://localhost:8002",
+            "access.token.claim": "true",
+            "id.token.claim": "false"
+          }
+        }
+      ]
     }
   ],
   "components": {
@@ -791,7 +825,8 @@
     "profile",
     "email",
     "roles",
-    "web-origins"
+    "web-origins",
+    "default-audience"
   ],
   "defaultOptionalClientScopes": [
     "offline_access",