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>

Dockerfile

@@ -1,9 +1,13 @@
 FROM node:18-alpine
 
+# Build arguments for versioning
+ARG GIT_COMMIT=unknown
+ARG APP_VERSION=1.0.0
+
 WORKDIR /app
 
-# Install curl for healthcheck
-RUN apk add --no-cache curl
+# Install curl for healthcheck and git for commit info
+RUN apk add --no-cache curl git
 
 # Copy package files first for layer caching
 COPY package*.json ./
@@ -12,6 +16,16 @@ RUN npm install --production
 # Copy app code (changes more often — below the cached layer)
 COPY . .
 
+# Try to get git commit if not provided and .git exists
+RUN if [ "$GIT_COMMIT" = "unknown" ] && [ -d .git ]; then \
+      GIT_COMMIT=$(git rev-parse HEAD 2>/dev/null || echo "unknown"); \
+    fi && \
+    echo "export GIT_COMMIT=${GIT_COMMIT}" >> /etc/profile.d/git-version.sh
+
+# Set environment variables for runtime
+ENV GIT_COMMIT=${GIT_COMMIT} \
+    APP_VERSION=${APP_VERSION}
+
 EXPOSE 3000
 
 HEALTHCHECK --interval=10s --timeout=3s --start-period=5s --retries=2 \