Implement robust auto-update system for GuruConnect agent

Features:
- Agent checks for updates periodically (hourly) during idle
- Admin can trigger immediate updates via dashboard "Update Agent" button
- Silent updates with in-place binary replacement (no reboot required)
- SHA-256 checksum verification before installation
- Semantic version comparison

Server changes:
- New releases table for tracking available versions
- GET /api/version endpoint for agent polling (unauthenticated)
- POST /api/machines/:id/update endpoint for admin push updates
- Release management API (/api/releases CRUD)
- Track agent_version in machine status

Agent changes:
- New update.rs module with download/verify/install/restart logic
- Handle ADMIN_UPDATE WebSocket command for push updates
- --post-update flag for cleanup after successful update
- Periodic update check in idle loop (persistent agents only)
- agent_version included in AgentStatus messages

Dashboard changes:
- Version display in machine detail panel
- "Update Agent" button for each connected machine

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

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

server/src/db/mod.rs

@@ -8,6 +8,7 @@ pub mod sessions;
 pub mod events;
 pub mod support_codes;
 pub mod users;
+pub mod releases;
 
 use anyhow::Result;
 use sqlx::postgres::PgPoolOptions;
@@ -19,6 +20,7 @@ pub use sessions::*;
 pub use events::*;
 pub use support_codes::*;
 pub use users::*;
+pub use releases::*;
 
 /// Database connection pool wrapper
 #[derive(Clone)]