Add /version endpoint with build metadata

- /version returns JSON with build_date and git_commit from env vars
- NixOS module injects GIT_COMMIT (first 8 chars of self.rev)
  and BUILD_DATE (self.lastModified) into systemd service environment

flake.nix

@@ -28,6 +28,8 @@
         let
           cfg = config.services.st-pierre;
           st-pierre-pkg = self.packages.${pkgs.stdenv.hostPlatform.system}.default;
+          gitHash = if (self ? rev) then builtins.substring 0 8 self.rev else "dirty";
+          lastUpdated = if (self ? lastModified) then builtins.toString self.lastModified else "0";
         in
         {
           options.services.st-pierre = {
@@ -141,6 +143,8 @@
                   EMBEDDING_BASE_URL = cfg.embeddingBaseUrl;
                   EMBEDDING_MODEL = cfg.embeddingModel;
                   SQLITE_PATH = builtins.toString cfg.sqlitePath;
+                  GIT_COMMIT = gitHash;
+                  BUILD_DATE = lastUpdated;
                 }
                 // lib.optionalAttrs (cfg.openaiApiKey != null) {
                   OPENAI_API_KEY = cfg.openaiApiKey;