feat(version): answer the build commit and time

GET /api/version is public. CI passes GIT_SHA and BUILD_TIME as Docker build args; a local build answers dev.

readme.md

@@ -93,6 +93,7 @@ accepted only by `GET /api/public/profiles/library` in the `X-Profile-Ticket` he
 | `GET` | `/api/library/sync-status`, `/summary`, `/activity`, `/wishlist`, `/steam-profile`, `/games-count` | JWT |
 | `GET` | `/api/public/profiles/library` | profile ticket |
 | `GET` | `/api/stats/public` | public |
+| `GET` | `/api/version` (`{commit, builtAt}` of the running image, from the CI build; `dev` locally) | public |
 
 ## Tech stack
 
@@ -118,6 +119,7 @@ flowchart LR
             LibApi["api"] --> LibService["service"] --> LibRepo["repo"]
         end
         Stats["stats"]
+        Version["version"]
         Steam["steam_adapter"]
         Igdb["igdb_adapter"]
         Speedrun["speedrun_adapter"]
@@ -140,6 +142,7 @@ flowchart LR
 |---|---|
 | `library` | Sync, backlog, manual games, playtime history, wishlist, achievements, public profile data |
 | `stats` | Public numbers for the landing page |
+| `version` | `GET /api/version`: commit and build time of the running image |
 | `steam_adapter` | Steam Web API client, availability check of Steam CDN pictures |
 | `igdb_adapter` | IGDB client (Twitch OAuth): time to beat, game tags for moods, release years, game search |
 | `speedrun_adapter` | speedrun.com client: game search by name, verified runs |