show build on website footer

PROJECT_KNOWLEDGE.md

@@ -20,6 +20,7 @@ Backend:
 - `/api/health` returns health/version information.
 - `/api/user` returns the authenticated user.
 - Access events are appended to `access_log.json`.
+- `build_info.json` is loaded at startup and exposed to all templates as `build_info`.
 
 Frontend:
 
@@ -28,6 +29,26 @@ Frontend:
 - `window.ARTICLE_DATA` is loaded from `/static/article-data.json`.
 - `window.APP_CONFIG` is currently defined inline for forklift rules.
 - A disclaimer modal blocks use until the user checks the confirmation checkbox.
+- The footer displays build metadata: version, branch, commit, and timestamp.
+
+## Build Info
+
+Build/deployment metadata is read from:
+
+```text
+build_info.json
+```
+
+Expected fields:
+
+- `version`
+- `branch`
+- `commit`
+- `timestamp`
+
+If `build_info.json` is missing, malformed, or does not contain a usable value, RollCalc falls back to `unknown` for that value and continues serving the site.
+
+The data is loaded centrally in `app.py` and made available to every template as `build_info`.
 
 ## Roll Geometry