feat(projects): M1 — the multi-project schema spine (§22)

Lays the additive foundation for hosting N projects per deployment, with
today's single corpus as the N=1 case. No behavior change: the app runs
exactly as before, single project, with the spine underneath.

- migration 025: `projects` + `project_members` tables; seed the `default`
  project (visibility=public, preserving open-by-default); thread
  `project_id NOT NULL DEFAULT 'default'` onto all 19 slug-bearing tables,
  backfilling existing rows. Additive — no table rebuilds; the slug-keyed
  uniqueness/PK rework is enumerated in the migration header and deferred to
  the slice that activates project #2.
- app/projects.py: §22.13 startup backfill of the default project's
  content_repo from META_REPO (idempotent — never clobbers a value the
  future registry mirror sets).
- config: REGISTRY_REPO wired (optional through M1; consumed by the M3
  mirror), documented in .env.example as META_REPO's successor.
- tests: 6 vertical assertions on the spine (seed, backfill, column shape,
  role CHECK, idempotency, config). Full suite 381 passed.
- docs: align Part C M1/M3 boundaries with the landed code (registry mirror
  + redirect move to M3).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/main.py

@@ -28,6 +28,7 @@ from . import (
     invites as invites_mod,
     otc,
     passcode as passcode_mod,
+    projects,
     providers as providers_mod,
     ratelimit,
     turnstile,
@@ -99,6 +100,7 @@ async def lifespan(app: FastAPI):
     config = load_config()
     db.run_migrations(config)
     db.init(config)
+    projects.seed_default_project(config)  # §22.13 step 1
     gitea = Gitea(config)
     bot = Bot(gitea)
     reconciler = cache.Reconciler(config, gitea)