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>

frontend/src/api/inception.ts

@@ -1,9 +1,14 @@
 /** Project inception (milestone 297): what a project was decided to inherit. */
 import { apiGet, apiPost } from "@/api/client";
+import type { ProjectPlatform } from "@/api/platforms";
 
 export interface InceptionChoices {
   design_system_id: number | null;
   seed_systems: boolean;
+  /** Platform slugs (milestone 463). null = not answered; a list is the
+   *  whole answer — members left out of it are recorded as "not this".
+   *  Absent on records decided before the question existed. */
+  platforms?: string[] | null;
 }
 
 export interface InceptionRecord {
@@ -17,16 +22,24 @@ export interface InceptionDefaults {
   design_system_id: number | null;
   design_systems: { id: number; title: string }[];
   systems: number;
+  platforms: { slug: string; name: string }[];
+  /** What detection (or an earlier answer) already says about the project. */
+  project_platforms: ProjectPlatform[];
 }
 
 export interface InceptionDecision {
   project_id: number;
   inception: InceptionRecord;
-  effects: { design_system_id: number | null; systems_seeded: string[] };
+  effects: {
+    design_system_id: number | null;
+    systems_seeded: string[];
+    /** The project's answers after the decision; null when left unstated. */
+    platforms: ProjectPlatform[] | null;
+  };
 }
 
 export const emptyChoices = (): InceptionChoices => ({
-  design_system_id: null, seed_systems: false,
+  design_system_id: null, seed_systems: false, platforms: null,
 });
 
 export const fetchInceptionDefaults = (projectId: number) =>