feat(m6): missions + snapshot CRUD, membership visibility, status state machine

Adds the mission layer that materialises template snapshots, plus the SPA
list / 3-step wizard / detail page.

Backend:
- app/services/missions.py — create_mission snapshots scenarios, tests, MITRE
  tags in a 4-query write; list/get apply a non-admin membership filter that
  collapses to 404 (no existence leak); status state machine enforces
  draft → in_progress → completed → archived with archived as a sink; the
  non-admin creator is auto-added as role_hint='red' to retain visibility.
- app/api/missions.py — 8 endpoints (list, get, create, update, add
  scenarios, set members, transition, soft-delete) with strict pydantic
  schemas. The transition endpoint splits the perm gate manually so
  archive requires mission.archive while other targets use mission.update.
- app/api/users.py — new GET /users/roster returning (id, email,
  display_name) only, gated by user.read OR mission.create OR
  mission.update — lets non-admin wizard users see assignable peers
  without exposing the admin /users payload.
- app/api/diag.py — /diag/reset truncates the mission_* tables before the
  template tables because the source_*_template_id FKs are ON DELETE SET
  NULL, which is cheaper to short-circuit by removing the children first.

Frontend:
- lib/missions.ts — typed client, queryKey factory, status accent map.
- pages/MissionsListPage.tsx — list cards with status accent + filters
  (q, client, status).
- pages/MissionsCreatePage.tsx — 3-step wizard (meta → scenarios → members)
  with member roster fed by /users/roster.
- pages/MissionDetailPage.tsx — header + transition buttons (legal next
  states only) + Tests/Members/Synthesis/Export tabs.
- Routes + nav entry (visible to anyone with mission.read or admin).

Tests:
- backend/tests/test_missions.py — 22 pytest covering snapshot fidelity,
  MITRE propagation, membership visibility, transition state machine,
  perm gating, member set replace, append scenarios, soft-delete, partial
  update, inverted-date rejection.
- e2e/tests/m6-missions.spec.ts — 5 Playwright (snapshot freezing, non-admin
  visibility, status transitions + 409, SPA wizard end-to-end, list filter).

Docs:
- CHANGELOG, tasks/testing-m6.md, tasks/lessons.md (snapshot tradeoffs,
  membership=404 pattern, /diag/reset order, auto-creator add).
- README + tasks/todo.md updated.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api/v1.py

@@ -9,6 +9,7 @@ from app.api.diag import bp as diag_bp
 from app.api.groups import bp as groups_bp
 from app.api.health import bp as health_bp
 from app.api.invitations import bp as invitations_bp
+from app.api.missions import bp as missions_bp
 from app.api.mitre import bp as mitre_bp
 from app.api.permissions import bp as permissions_bp
 from app.api.scenario_templates import bp as scenario_templates_bp
@@ -28,3 +29,4 @@ bp.register_blueprint(permissions_bp)
 bp.register_blueprint(mitre_bp)
 bp.register_blueprint(test_templates_bp)
 bp.register_blueprint(scenario_templates_bp)
+bp.register_blueprint(missions_bp)