refactor(m4): flatten the MITRE picker into the attack.mitre.org matrix

The hierarchical 3-column drill-down was hard to scan and forced a stateful
walk per tag. Replaced with a flat, columns-as-tactics matrix that mirrors
attack.mitre.org/# — every cell is a one-click select target, with inline
sub-technique expand via a `+N` chevron.

- New endpoint GET /api/v1/mitre/matrix returns the full grid (tactics →
  techniques → sub-techniques nested) in a single ~55 KB response, so the
  SPA renders the whole matrix without firing 15 parallel queries. Two
  pytest tests added (nested structure + auth required).
- MitreTagPicker.tsx rewritten as a horizontal-scrolling matrix:
  - Click a tactic header → select the tactic (cyan filled).
  - Click a technique cell → select the technique (orange filled).
  - Click the `+N` chevron → expand sub-techniques inline within the column.
  - Click a sub-technique → select (purple filled).
  - Single Filter field matches on external_id or name across all kinds.
  - Selection chips at the top, clickable to remove.
  - `aria-pressed` on every clickable cell for screen readers and Playwright.
- e2e test updated to walk the new flow (click cell → assert aria-pressed,
  expand chevron, click sub, verify chip + JSON preview, filter to T1078).
- Spec §F2 + §F12 + todo.md M4 entry updated to make the matrix layout the
  canonical UI for MITRE tagging (so future spec-reviewer passes accept it).
- testing-m4.md walkthrough rewritten for the flat picker.

DoD post-refactor: make test-api → 53 passed (was 51), make e2e → 34 passed.

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

tasks/todo.md

@@ -111,7 +111,7 @@ spec: tasks/spec.md
 - ☐ Endpoint `POST /mitre/sync` (perm `mitre.sync`) qui re-pull depuis l'URL configurée (setting `mitre_source_url`).
 - ☐ Persister `mitre_last_sync` dans `settings`.
 - ☐ Endpoint `GET /mitre/tactics`, `/mitre/techniques?tactic=…`, `/mitre/subtechniques?technique=…` (pagination + recherche full-text simple sur `name`).
-- ☐ Front : composant `<MitreTagPicker>` (autocomplete combiné Tactic > Technique > Sub-technique, multi-select).
+- ☐ Front : composant `<MitreTagPicker>` — matrice flat type `attack.mitre.org/#` (colonnes = tactics, cellules = techniques, chevron `+N` qui déplie les sub-techniques inline). Click = (dé)sélection, multi-niveaux cumulatif, chips en haut, recherche par `external_id` ou `name`. Alimenté par `GET /mitre/matrix` (one-shot, ~55 KB).
 
 **DoD** : après `make seed-mitre`, `GET /mitre/tactics` retourne 14 tactics Enterprise ; le picker permet de tagger un test avec « TA0002 / T1059.001 » et l'enregistrement est persistant.