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/api/mod.rs

@@ -2,6 +2,7 @@
 
 pub mod auth;
 pub mod users;
+pub mod releases;
 
 use axum::{
     extract::{Path, State, Query},
@@ -48,6 +49,7 @@ pub struct SessionInfo {
     pub is_elevated: bool,
     pub uptime_secs: i64,
     pub display_count: i32,
+    pub agent_version: Option<String>,
 }
 
 impl From<crate::session::Session> for SessionInfo {
@@ -67,6 +69,7 @@ impl From<crate::session::Session> for SessionInfo {
             is_elevated: s.is_elevated,
             uptime_secs: s.uptime_secs,
             display_count: s.display_count,
+            agent_version: s.agent_version,
         }
     }
 }