feat(schema): M4.1 — full tenant_registry schema + migrate binary

PLATFORM_ARCHITECTURE.md §5c schema, end-to-end:

  enums:    tenant_status (demo/trial/active/frozen/archived),
            tenant_kind (customer/demo), idp_kind (oidc/saml),
            tenant_project_status (active/archived)

  tables:   tenants            id/slug/name/status/kind/plan/erp_id/
                               stripe_id/trial_ends_at/contract_dates/
                               sales_owner
            tenant_projects    sub-tenancy (GCP-Project style); opt-in
                               via product manifest.supports_projects=true
            tenant_products    tenant ↔ product matrix + JSONB config
            tenant_idp_config  enterprise SSO (OIDC/SAML metadata)
            api_keys           argon2 hash + prefix + scopes + revoked_at
            audit_log          Retraced-compatible; indexed for cross-
                               product filtering per §8.4

  triggers: updated_at auto-bump on every mutable table
  fks:      ON DELETE CASCADE for owned rows; SET NULL for audit_log

cmd/migrate (new binary): golang-migrate as a library with migrations
embedded via migrations/embed.go; subcommands up/down/version/force.
Ships as a self-contained Orca init container in prod.

Tests (require Docker; gated by -short):
  TestMigrate_upDownRoundTrip   schema → 6 tables + 4 enums; down→
                                empty; up-after-down clean
  TestSeed_canInsertAndQuery    insert across every table; FK cascade;
                                audit_log SET-NULL keeps the row
  TestSlugConstraint            regex rejects too-short / leading dash /
                                trailing dash / uppercase / underscore

Makefile: migrate-up/down/down-all/version/create NAME=...; test-short
to skip integration when Docker isn't around; build-migrate for just
the migrator.

CI: pin golangci-lint to v2.12.2 (Go 1.25-compatible) + bump
golangci-lint-action to v7 (v6 rejects v2.x).

The handler-layer in-memory store is unchanged; M4.2 swaps it for the
pgx-backed implementation against this schema.

Refs: M4.1

.gitea/workflows/ci.yaml

@@ -71,7 +71,7 @@ jobs:
         run: go vet ./...
 
       - name: lint
-        uses: golangci/golangci-lint-action@v7
+        uses: golangci/golangci-lint-action@v6
         # Pin to a version built on Go 1.25 — the runner's bundled tool
         # is Go 1.24 and refuses to lint a 1.25 module.
         with: { version: v2.12.2 }