feat(family): platforms declared at inception and detected from bound repos (milestone 463 step 2, #4988)

A project's platforms decide which family ideas reach it. This step makes membership answerable from every door:

- services/platforms.py: the global catalog (writes are admin-only and duplicate-gated by slug); pure marker detection; and membership reads and writes. Detection only ADDS, and only where nobody has answered. It never overrides a declared or rejected row and never removes one.
- coverage: the archive scan now carries every path, and the refresh runs detection fail-open.
- inception: a platforms choice (slugs, or null for unanswered). The list is the whole answer: members left out of it become rejected.
- MCP: list_platforms and set_project_platforms; enter_project and get_project carry the project's platforms.
- REST: /api/platforms (admin writes) and /api/projects/<id>/platforms.
- UI: a platforms checklist on the inception card, a Family tab on ProjectView, and a Platforms admin tab in Settings.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/app.py

@@ -32,6 +32,7 @@ from scribe.routes.trash import trash_bp
 from scribe.routes.dashboard import dashboard_bp
 from scribe.routes.systems import systems_bp
 from scribe.routes.canonical_systems import canonical_systems_bp
+from scribe.routes.platforms import platforms_bp
 from scribe.routes.lessons import lessons_bp
 from scribe.routes.snippets import snippets_bp
 from scribe.routes.webhooks import webhooks_bp
@@ -101,6 +102,7 @@ def create_app() -> Quart:
     app.register_blueprint(dashboard_bp)
     app.register_blueprint(systems_bp)
     app.register_blueprint(canonical_systems_bp)
+    app.register_blueprint(platforms_bp)
     app.register_blueprint(snippets_bp)
     app.register_blueprint(webhooks_bp)