Drop docs/deployment-plan.md (internal planning doc, kept out of the repo) and
replace the personal lab DB IP and host names in the env/compose/deploy docs with
generic placeholders, so the repo is safe to make public later.
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.
