feat(web): show git SHA in footer for post-deploy verification

Dockerfile takes SOURCE_COMMIT as a build arg and bakes it into the
image as GIT_COMMIT. Coolify sets SOURCE_COMMIT on every deploy, so
the value in the footer changes with each successful push → build.

ARG is placed after COPY . so only a thin final layer rebuilds when
the SHA changes; pip install stays cached. Outside Coolify the
default is "dev" and the footer renders "build dev".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -1,6 +1,9 @@
 services:
   web:
-    build: ./web
+    build:
+      context: ./web
+      args:
+        SOURCE_COMMIT: ${SOURCE_COMMIT:-dev}
     container_name: lazyflat-web
     restart: unless-stopped
     depends_on: