Point email links at conchecker.scoutitsystems.com and show build SHA in header.

APP_BASE_URL default (config, .env.example, both compose files) is now
https://conchecker.scoutitsystems.com with no port, so review-required
and final-report email links use the public site. CI bakes the short
commit SHA into the image as APP_BUILD via a Docker build-arg; /health
returns version+build and the site header shows the build so it's easy
to confirm which image is deployed. Local runs default to 'dev'.

backend/config.py

@@ -123,7 +123,10 @@ APP_VERSION = "0.1.0"
 # Public base URL used to build the "view results" link in notification
 # emails. Set to whatever address users reach this server on (e.g. the
 # Tailscale/LAN URL) so the link in the email actually resolves.
-APP_BASE_URL = os.getenv("APP_BASE_URL", "http://localhost:8099")
+APP_BASE_URL = os.getenv("APP_BASE_URL", "https://conchecker.scoutitsystems.com")
+# Build identifier baked into the Docker image by CI (sha-<short_sha>, matching
+# the image tag). Shown in the site header and /health. "dev" for local runs.
+APP_BUILD = os.getenv("APP_BUILD", "dev")
 
 # -- Email / SMTP (optional notification on completion) -------------
 # If unset, the app still works; it just logs "SMTP not configured" and