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_inception.py

@@ -27,7 +27,7 @@ def test_project_carries_an_inception_record_and_to_dict_shows_it():
 
 
 def test_validate_inception_pins_the_choice_vocabulary():
-    assert CHOICE_KEYS == ("design_system_id", "seed_systems")
+    assert CHOICE_KEYS == ("design_system_id", "seed_systems", "platforms")
     assert validate_inception({}) is None
     assert validate_inception({"design_system_id": 3, "seed_systems": True}) is None
     assert validate_inception({"design_system_id": None}) is None
@@ -46,10 +46,28 @@ def test_validate_inception_pins_the_choice_vocabulary():
     assert "true or false" in validate_inception({"seed_systems": "yes"})
 
 
+def test_validate_inception_takes_platforms_as_slugs_or_unstated():
+    """Slugs, not ids: an inception record outlives a restore, and a slug is
+    what both sides of a restore agree on. None is "not answered", which is
+    different from [] — "none of them"."""
+    assert validate_inception({"platforms": ["android-app", "go"]}) is None
+    assert validate_inception({"platforms": []}) is None
+    assert validate_inception({"platforms": None}) is None
+    assert "platform slugs" in validate_inception({"platforms": "android-app"})
+    assert "platform slugs" in validate_inception({"platforms": [3]})
+    assert "platform slugs" in validate_inception({"platforms": ["  "]})
+
+
 def test_normalize_choices_is_canonical_and_complete():
     out = normalize_choices({"design_system_id": 4})
-    assert out == {"design_system_id": 4, "seed_systems": False}
-    assert normalize_choices(None) == {"design_system_id": None, "seed_systems": False}
+    assert out == {"design_system_id": 4, "seed_systems": False, "platforms": None}
+    assert normalize_choices(None) == {
+        "design_system_id": None, "seed_systems": False, "platforms": None,
+    }
+    # Sorted and de-duplicated, so two equal answers record identically.
+    assert normalize_choices({"platforms": ["go", " android-app", "go"]})["platforms"] == [
+        "android-app", "go",
+    ]
 
 
 def test_standard_systems_vocabulary_reads_the_catalog_not_a_constant():