feat(multiuser): phase 2, multi-user auth pipeline

Adds a parallel multi-user auth branch (the single-user Basic-auth path is
left byte-identical). Off unless CODEMAN_MULTIUSER/--multiuser.

- middleware/auth.ts: mode-selecting registerAuthMiddleware. New async
  multi-user hook verifies username:password against the user store (scrypt),
  mints identity-carrying cookies, decorates req.authUser, enforces a per-IP
  AND per-username failure bucket, and the mustChangePassword lockbox. The
  hook-secret loopback bypass is now a single shared helper used by both
  branches. FastifyRequest.authUser module augmentation.
- ports/auth-port.ts: AuthSessionRecord gains username/role/mustChangePassword.
- user-store.ts: verifyPassword (timing-equalized against user enumeration).
- route-helpers.ts: getAuthUser (synthetic admin fallback), canAccessOwned,
  requireAdmin, revokeUserSessions; findSessionOrFail gains an optional req for
  a NOT_FOUND owner check (dormant until phase 3 wires callers).
- routes/me-routes.ts: GET /api/me (synthetic admin in single-user) and
  POST /api/me/password (verify current, min 8, clear mustChangePassword,
  revoke other sessions).
- QR: QrTokenRecord + AuthSessionRecord carry a username; tunnel-manager
  mintUserToken / consumeTokenWithIdentity / getQrSvgForCode; /q/:code binds
  the cookie to the token's user (rejects identity-less tokens in multi-user);
  GET /api/tunnel/qr mints a per-user token. Single-user keeps the rotating token.
- server.ts: bootstrap the initial admin from CODEMAN_USERNAME/PASSWORD on first
  boot (refuse to start with no users); multi-user with >= 1 user satisfies the
  non-loopback auth requirement and the tunnel-enable guard; userFailures bucket
  disposal.
- types/api.ts: FORBIDDEN, PASSWORD_CHANGE_REQUIRED, USER_EXISTS, USER_NOT_FOUND,
  LAST_ADMIN error codes (message + status wired).
- Session.owner field + getter/setter, SessionState.owner, MuxSession.owner,
  CreateSessionOptions.owner (foundation for phase 3 ownership threading).

Tests: test/multiuser-auth.test.ts (10, live server on 3170/3171). Existing auth
suite (auth-security, qr-auth, cod54-hook-event, network-auth-policy) unchanged
and green; full test:ci sweep passes (3519 tests).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/tunnel-manager.ts

@@ -43,6 +43,8 @@ interface QrTokenRecord {
   shortCode: string; // 6 chars base62 (for URL path)
   createdAt: number; // Date.now()
   consumed: boolean; // single-use flag
+  /** Multi-user: the user this token logs in when redeemed (absent = rotating global token). */
+  username?: string;
 }
 
 /** Rejection-sampled base62 short code — no modulo bias */
@@ -378,23 +380,64 @@ export class TunnelManager extends EventEmitter {
    * Map.get() is hash-based — no timing side-channel from string comparison.
    */
   consumeToken(shortCode: string): boolean {
+    return this.consumeTokenWithIdentity(shortCode).ok;
+  }
+
+  /**
+   * Like consumeToken, but also returns the bound username for multi-user tokens
+   * (undefined for the rotating global token). Only the identity-less rotating
+   * token triggers an immediate re-rotation (desktop gets a fresh QR); per-user
+   * tokens are on-demand and self-expire.
+   */
+  consumeTokenWithIdentity(shortCode: string): { ok: boolean; username?: string } {
     // Global rate limit (across all IPs)
-    if (this.qrAttemptCount >= QR_RATE_LIMIT_MAX) return false;
+    if (this.qrAttemptCount >= QR_RATE_LIMIT_MAX) return { ok: false };
     this.qrAttemptCount++;
 
     const record = this.qrTokensByCode.get(shortCode);
-    if (!record) return false;
-    if (record.consumed) return false;
+    if (!record) return { ok: false };
+    if (record.consumed) return { ok: false };
 
     const now = Date.now();
-    if (now - record.createdAt > QR_TOKEN_GRACE_MS) return false;
+    if (now - record.createdAt > QR_TOKEN_GRACE_MS) return { ok: false };
 
     // Atomic consume (single-threaded JS = no race)
     record.consumed = true;
-    // Immediately rotate so desktop gets a fresh QR
-    this.rotateToken();
-    this.emit('qrTokenRegenerated');
-    return true;
+    const username = record.username;
+    if (!username) {
+      // Rotating global token — immediately rotate so desktop gets a fresh QR.
+      this.rotateToken();
+      this.emit('qrTokenRegenerated');
+    } else {
+      this.qrTokensByCode.delete(shortCode);
+    }
+    return { ok: true, username };
+  }
+
+  /**
+   * Multi-user: mint a single-use token bound to a specific user (on-demand, no
+   * rotation). Evicts expired/consumed tokens first. Returns the short code.
+   */
+  mintUserToken(username: string): string {
+    const now = Date.now();
+    for (const [code, rec] of this.qrTokensByCode) {
+      if (now - rec.createdAt > QR_TOKEN_GRACE_MS || rec.consumed) this.qrTokensByCode.delete(code);
+    }
+    const record: QrTokenRecord = {
+      token: randomBytes(32).toString('hex'),
+      shortCode: generateShortCode(),
+      createdAt: Date.now(),
+      consumed: false,
+      username,
+    };
+    this.qrTokensByCode.set(record.shortCode, record);
+    return record.shortCode;
+  }
+
+  /** Render a QR SVG for an arbitrary short code (used by per-user minting). */
+  async getQrSvgForCode(tunnelUrl: string, code: string): Promise<string> {
+    const QRCode = await import('qrcode');
+    return QRCode.toString(`${tunnelUrl}/q/${code}`, { type: 'svg', margin: 2, width: 256 });
   }
 
   /** Force-regenerate (manual revocation via API) */