fix(§22): heal mig-029 vs registry-mirror collection-id divergence

On a deployment that already had >=2 projects when migration 029 ran, 029
seeds the default project's collection id as the project id (e.g. 'ohm'),
but the registry mirror expects 'default' -> it inserted a duplicate empty
'default' collection, orphaning the entries.

Adds projects.reconcile_default_collection_id() -- the collection-grain twin
of restamp_default_project -- run at startup BEFORE the mirror so it merges
onto the canonical 'default' collection instead of duplicating. Renames the
divergent collection id and cascades collection_id across all keyed tables
(FK-off atomic rename + foreign_key_check). Idempotent; no-op on fresh /
single-project / already-aligned deploys. Seam tests show the duplicate forms
without the fix and merges with it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/main.py

@@ -102,6 +102,12 @@ async def lifespan(app: FastAPI):
     db.run_migrations(config)
     db.init(config)
     gitea = Gitea(config)
+    # §22 framework heal: reconcile a divergent default-project collection id
+    # (migration 029's ≥2-projects seed names it after the project, e.g. 'ohm',
+    # but the mirror expects 'default') BEFORE the mirror runs, so the mirror
+    # merges onto the canonical 'default' collection instead of duplicating it.
+    # Idempotent no-op on fresh / single-project / already-aligned deployments.
+    projects.reconcile_default_collection_id(config)
     # §22.2: mirror the registry before anything reads projects/content_repo.
     # First boot has no last-good rows, so a missing/invalid registry is fatal
     # (loud-fail per separation-of-concerns); the reconciler sweep keeps it