security(oauth): harden OAuth 2.1/MCP implementation (Critical + High + Medium findings)

Address 14 security findings from internal review of the OAuth 2.1 + MCP layer:

Critical:
- C1: Scope-gate all MCP resources (trips, budget, packing, collab, atlas, vacay, etc.)
- C2: Wire token/session revocation into active MCP session lifecycle per (user, client_id)
- C3: Refresh-token replay detection via parent_token_id chain + cascade revoke on replay

High:
- H1: Validate PKCE code_challenge (43-char base64url) and code_verifier (43–128 chars) format
- H2: Rate-limit /oauth/token (30/min), /authorize/validate (30/min), /oauth/revoke (10/min)
- H3: Strip client metadata from unauthenticated /authorize/validate responses (oracle prevention)
- H4: Constant-time secret comparison via crypto.timingSafeEqual (prevents timing attacks)
- H5: Collapse all invalid_grant cases to a single generic message; log specifics server-side

Medium:
- M1: Set Cache-Control: no-store + Pragma: no-cache on token endpoint responses
- M2: Return 404 (not 200/403) on discovery + revoke endpoints when MCP addon is disabled
- M4: Audit-log all OAuth lifecycle events (create, consent, issue, refresh, revoke, replay)
- M5: Union consent scopes on re-authorization instead of replacing existing grants
- M7: Require httpOnly cookie auth (not Bearer JWT) on all state-mutating OAuth endpoints
- M8: Strict Bearer scheme check in MCP token verification

Refactoring:
- Extract MCP session management (sessions Map, revokeUserSessions, revokeUserSessionsForClient)
  into mcp/sessionManager.ts to break the circular dependency between oauthService and mcp/index
- Extract verifyJwtAndLoadUser helper in auth middleware, shared by authenticate and new
  requireCookieAuth middleware

Tests:
- Fix all existing integration tests broken by the security hardening (OAUTH-019 to OAUTH-032)
- Add 13 new integration tests covering M1, M2, H1, H3, H5, M5, M7, C3
- Add 14 new unit tests covering C2, C3, H1, H3, M5 behaviors in oauthService

server/src/db/migrations.ts

@@ -926,6 +926,13 @@ function runMigrations(db: Database.Database): void {
         CREATE UNIQUE INDEX IF NOT EXISTS idx_oauth_tokens_refresh ON oauth_tokens(refresh_token_hash);
       `);
     },
+    // Migration: Refresh-token rotation chain tracking for replay detection
+    () => {
+      db.exec(`
+        ALTER TABLE oauth_tokens ADD COLUMN parent_token_id INTEGER REFERENCES oauth_tokens(id);
+        CREATE INDEX IF NOT EXISTS idx_oauth_tokens_parent ON oauth_tokens(parent_token_id);
+      `);
+    },
   ];
 
   if (currentVersion < migrations.length) {