Implement GuruRMM Phase 1: Real-time tunnel infrastructure

Complete bidirectional tunnel communication between server and agents,
enabling persistent secure channels for future command execution and
file operations. Agents transition from heartbeat mode to tunnel mode
on-demand while maintaining WebSocket connection.

Server Implementation:
- Database layer (db/tunnel.rs): Session CRUD, ownership validation,
  cleanup on disconnect (prevents orphaned sessions)
- API endpoints (api/tunnel.rs): POST /open, POST /close, GET /status
  with JWT auth, UUID validation, proper HTTP status codes
- Protocol extension (ws/mod.rs): TunnelOpen/Close/Data messages,
  agent response handlers (TunnelReady/Data/Error)
- Migration (006_tunnel_sessions.sql): tech_sessions table with
  partial unique constraint, foreign keys with CASCADE, audit table

Agent Implementation:
- State machine (tunnel/mod.rs): AgentMode (Heartbeat ↔ Tunnel),
  channel multiplexing, concurrent session prevention
- WebSocket handlers (transport/websocket.rs): Open/close tunnel,
  mode switching without dropping connection, cleanup on disconnect
- Protocol extension (transport/mod.rs): TunnelReady/Data/Error
  messages matching server definitions
- Unit tests: Lifecycle and channel management coverage

Key Features:
- Security: JWT auth, session ownership verification, SQL injection
  prevention, constraint-based duplicate session blocking
- Cleanup: Automatic session closure on agent disconnect (both sides),
  channel cleanup, graceful state transitions
- Error handling: Proper HTTP status codes (400/403/404/409/500),
  comprehensive Result types, detailed logging
- Extensibility: Channel types ready (Terminal/File/Registry/Service),
  TunnelDataPayload enum for Phase 2+ expansion

Phase 1 Scope (Implemented):
- Tunnel session lifecycle management
- Mode switching (heartbeat ↔ tunnel)
- Protocol message routing
- Database session tracking

Phase 2 Next Steps:
- Terminal command execution (tokio::process::Command)
- Client WebSocket connections for output streaming
- Command audit logging
- File transfer operations

Verification:
- Server compiles successfully (0 errors)
- Agent unit tests pass (tunnel lifecycle, channel management)
- Code review approved (protocol alignment verified)
- Database constraints enforce referential integrity
- Cleanup tested (session closure on disconnect)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

projects/msp-tools/guru-rmm/server/migrations/006_tunnel_sessions.sql

@@ -6,7 +6,7 @@
 CREATE TABLE tech_sessions (
     id SERIAL PRIMARY KEY,
     session_id VARCHAR(36) UNIQUE NOT NULL,
-    tech_id INTEGER NOT NULL,
+    tech_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
     agent_id UUID NOT NULL REFERENCES agents(id) ON DELETE CASCADE,
     opened_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
     last_activity TIMESTAMPTZ NOT NULL DEFAULT NOW(),