Git workflow, environments & CI/CD pipeline (#1)

docker-compose.staging.yml

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+# Staging overlay for InboxIntel.
+#
+# The base docker-compose.yml IS the production definition (Linux containers,
+# ASPNETCORE_ENVIRONMENT=Production). This overlay layers a *staging* variant on
+# top of it so you can run a production-shaped stack locally on Windows WITHOUT
+# clobbering a real production deployment's data, ports, or volumes.
+#
+# It differs from prod only in the ways staging is meant to differ:
+#   - the dev/test banner is on (App__DevMode=true)
+#   - the initial Gmail sync is capped so a big mailbox doesn't take forever
+#   - ports are shifted into the 18xxx range so staging can run alongside prod
+#   - a distinct project name gives it its own isolated pgdata + keys volumes
+#
+# Run it with the -p (project name) flag so volumes/networks are namespaced:
+#
+#   docker compose -p inboxintel-staging \
+#     --env-file deploy/.env.staging \
+#     -f docker-compose.yml -f docker-compose.staging.yml up --build -d
+#
+# (deploy/up.ps1 -Staging / up.sh --staging wrap this for you.)
+
+services:
+  postgres:
+    ports:
+      - "127.0.0.1:15432:5432"
+
+  api:
+    environment:
+      # Same Production runtime as prod (real config binding, real build), but
+      # flagged as a non-production instance so the UI shows the staging banner
+      # and the first sync is bounded.
+      App__DevMode: "true"
+      GmailSync__MaxMessages: ${MAX_MESSAGES:-2000}
+      Cors__Origins__0: ${FRONTEND_ORIGIN:-http://localhost:18081}
+    ports:
+      - "127.0.0.1:18080:8080"
+
+  frontend:
+    ports:
+      - "18081:80"
+
+  nginx:
+    ports:
+      - "18000:80"