feat(deploy): central Postgres + scheduler split for Proxmox 24/7 backfill

Add a server compose (full stack, Postgres on the LAN, scheduler on, host-visible
pgdata bind mount) and a localdev compose (webapp only, no DB, scheduler off) that
points at the central DB. Document the single-shared-database topology, the
exactly-one-scheduler rule, and YOUTUBE_API_KEY for unattended backfill that does
not depend on a 7-day OAuth refresh token.

docker-compose.localdev.yml

@@ -0,0 +1,30 @@
+# Local development against the central (Proxmox) Postgres.
+#
+# Runs only the webapp/API — no local database, no scheduler — pointed at the shared
+# DB on the server. You see the same live data the 24/7 backfill is filling, and you
+# never run a second scheduler against it.
+#
+# Setup: in .env set
+#   DATABASE_URL=postgresql+psycopg://subfeed:<password>@your-db-host:5432/subfeed
+# (use the central DB's host/port and the real POSTGRES_PASSWORD), then:
+#   docker compose -f docker-compose.localdev.yml up --build
+#
+# Browse at http://localhost:8080 — Google login works here because the OAuth redirect
+# is http://localhost:8080/auth/callback.
+#
+# Note: the API runs `alembic upgrade head` on startup, so launching this with newer
+# local migrations will apply them to the shared DB. For risky schema work, point
+# DATABASE_URL at a throwaway local Postgres instead.
+
+services:
+  api:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    env_file: .env
+    environment:
+      # Exactly one scheduler may write to the shared DB; the server owns it.
+      SCHEDULER_ENABLED: "false"
+    ports:
+      - "${APP_PORT:-8080}:8000"
+    restart: unless-stopped