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>

README.md

@@ -12,6 +12,7 @@ Collaborative purple-team platform. Red team logs the tests they execute (proced
 - **RBAC (M3+)**: atomic permissions (31 codes) bundled into custom groups; 3 system groups seeded (`admin` / `redteam` / `blueteam`).
 - **MITRE ATT&CK (M4+)**: Enterprise reference catalogue pinned to v19.0, seedable via `make seed-mitre`.
 - **Template catalogue (M5+)**: reusable `test_templates` (markdown procedure, OPSEC level, free tags, expected IOCs, MITRE tags) + ordered `scenario_templates` with drag-and-drop reordering. Admin pages at `/admin/tests` and `/admin/scenarios`.
+- **Missions (M6+)**: `missions` snapshot one or more scenario templates at creation time; template edits don't drift live missions (`mission_*` tables freeze every field, including MITRE tags). Non-admin members see only their own missions (membership filter, 404 on existence-leak attempts). Status state machine `draft → in_progress → completed → archived`, archive perm gated separately. SPA: list/filter at `/missions`, 3-step create wizard at `/missions/new`, detail page with Tests / Members / Synthesis / Export tabs.
 - **Delivery**: docker-compose. TLS termination is expected to be handled by an external reverse proxy in production.
 
 ## Quickstart
@@ -94,7 +95,7 @@ See `.env.example`. The most important ones:
 
 ## Testing
 
-- **Manual + automated checklist for the current milestone**: see [`tasks/testing-m<N>.md`](tasks/testing-m4.md) (current: `testing-m4.md`).
+- **Manual + automated checklist for the current milestone**: see [`tasks/testing-m<N>.md`](tasks/testing-m6.md) (current: `testing-m6.md`).
 - **Backend unit tests**: `make test-api`
 - **End-to-end (Playwright)**: `make e2e-install` (once), then `make up && make e2e`. Reports land in `e2e/playwright-report/` (HTML + JUnit XML); open with `make e2e-report`.
 
@@ -136,7 +137,7 @@ The hooks run `ruff` + `ruff-format` on the backend and `eslint` / `tsc --noEmit
 
 ## Roadmap
 
-See `tasks/todo.md`. Current milestone: **M4 — MITRE ATT&CK Enterprise**. Next: M5 (test & scenario templates).
+See `tasks/todo.md`. Current milestone: **M6 — Missions & snapshot** (done). Next: M7 (red/blue execution on a mission test).
 
 ## License