Add Git commit ID tracking to health endpoint

Features:
- Health endpoint now returns git_commit SHA and version
- Auto-detects git commit during Docker build
- Falls back to 'unknown' if git info unavailable
- Allows AI agents to verify deployed version

Changes:
1. server.js: Add git_commit and version to /health response
2. Dockerfile: Add git installation and commit detection
3. Dockerfile: Accept GIT_COMMIT and APP_VERSION build args
4. docker-compose.yml: Pass build args and env vars for versioning
5. README.md: Document version tracking in health endpoint

Example response:
{
  "status": "healthy",
  "postgres": "connected",
  "redis": "connected",
  "timestamp": "2026-02-14T10:30:00.000Z",
  "git_commit": "eb6b6e0a1b2c3d4e5f6g7h8i9j0k1l2m3n4o5p6q",
  "version": "1.0.0"
}

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

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

server.js

@@ -55,12 +55,16 @@ app.get('/health', async (req, res) => {
       status: 'healthy',
       postgres: 'connected',
       redis: 'connected',
-      timestamp: pgResult.rows[0].now
+      timestamp: pgResult.rows[0].now,
+      git_commit: process.env.GIT_COMMIT || 'unknown',
+      version: process.env.APP_VERSION || '1.0.0'
     });
   } catch (error) {
     res.status(500).json({
       status: 'unhealthy',
-      error: error.message
+      error: error.message,
+      git_commit: process.env.GIT_COMMIT || 'unknown',
+      version: process.env.APP_VERSION || '1.0.0'
     });
   }
 });