Add build info metric and diagnostic logging for Garmin sync

- Add phaseflow_build_info metric with version and commit labels
- Inject GIT_COMMIT env var at build time via next.config.ts
- Add logging to all Garmin fetch functions (HRV, body battery, intensity)
- Log API response status codes, actual data values, and errors

This enables verifying which build is deployed and diagnosing
silent failures where Garmin API returns errors but sync reports success.

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

next.config.ts

@@ -1,7 +1,22 @@
+// ABOUTME: Next.js configuration for PhaseFlow application.
+// ABOUTME: Configures standalone output and injects git commit hash for build verification.
+import { execSync } from "node:child_process";
 import type { NextConfig } from "next";
 
+// Get git commit hash at build time for deployment verification
+function getGitCommit(): string {
+  try {
+    return execSync("git rev-parse --short HEAD").toString().trim();
+  } catch {
+    return "unknown";
+  }
+}
+
 const nextConfig: NextConfig = {
   output: "standalone",
+  env: {
+    GIT_COMMIT: process.env.GIT_COMMIT || getGitCommit(),
+  },
 };
 
 export default nextConfig;