6f901e3e2c
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>