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>

tests/test_routes_platforms.py

@@ -0,0 +1,28 @@
+"""Structural tests for the platforms blueprint (milestone 463 step 2) —
+registration and the route-to-service contract. What the writes do is in
+tests/test_integration_platforms.py."""
+import inspect
+
+
+def test_platforms_blueprint_registered_in_app():
+    from scribe.app import create_app
+    app = create_app()
+    assert "platforms" in app.blueprints
+    rules = {(r.rule, m) for r in app.url_map.iter_rules() for m in r.methods}
+    for rule, method in (
+        ("/api/platforms", "GET"),
+        ("/api/platforms", "POST"),
+        ("/api/platforms/<int:platform_id>", "PATCH"),
+        ("/api/projects/<int:project_id>/platforms", "GET"),
+        ("/api/projects/<int:project_id>/platforms", "PUT"),
+    ):
+        assert (rule, method) in rules, f"{method} {rule} is not routed"
+
+
+def test_writes_to_the_catalog_and_to_a_project_take_the_caller():
+    """The catalog's admin gate and the project's write gate both live in the
+    service, so every write must be handed the caller to check."""
+    from scribe.services import platforms as svc
+    for name in ("create_platform", "update_platform", "set_project_platforms",
+                 "project_platforms"):
+        assert "user_id" in inspect.signature(getattr(svc, name)).parameters