ci(obs): document POSTGRES_HOST derivation from Compose project name

The container names archiv-staging-db-1 and archiv-production-db-1 are
derived from the Compose project name + service name. A project rename
silently breaks the obs stack DB connection. Add a comment at the point
of definition so the dependency is obvious when someone changes it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/nightly.yml

@@ -146,6 +146,8 @@ jobs:
           POSTGRES_PASSWORD=${{ secrets.STAGING_POSTGRES_PASSWORD }}
           POSTGRES_HOST=archiv-staging-db-1
           EOF
+          # Note: POSTGRES_HOST is derived from the Compose project name (archiv-staging)
+          # and service name (db). A project rename requires updating this value.
           chmod 600 /opt/familienarchiv/obs-secrets.env
 
       - name: Validate observability compose config

.gitea/workflows/release.yml

@@ -114,6 +114,8 @@ jobs:
           POSTGRES_PASSWORD=${{ secrets.PROD_POSTGRES_PASSWORD }}
           POSTGRES_HOST=archiv-production-db-1
           EOF
+          # Note: POSTGRES_HOST is derived from the Compose project name (archiv-production)
+          # and service name (db). A project rename requires updating this value.
           chmod 600 /opt/familienarchiv/obs-secrets.env
 
       - name: Validate observability compose config