feat(suggestions): full suggestion workflow with per-step notes

DB migration h5c6d7e8f9a0:
- Extends tdstatus enum: submitted → analyse → plan → implement →
  test → review → finished (+ wont_fix remains)
- New td_notes table: td_id FK (CASCADE), step, author, content, created_at

API:
- TDNote model + TDNoteCreate/TDNoteRead schemas
- TDRead includes notes[] (selectin loaded)
- New routes: GET/POST /technical-debt/{id}/notes/
- list_td status filter accepts str (all enum values)

Modal: new submissions use status="submitted" instead of "open"

UI Feedback page revamp:
- Visual step-by-step stepper (submitted→analyse→plan→implement→test→review→finished)
- Per-step notes: view all notes, add note inline
- Action buttons: advance to next step, won't fix
- Review step highlighted as awaiting original suggester confirmation
- Closed items (finished/wont_fix) shown with last 2 notes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dashboard/src/components/improvement-modal.js

@@ -342,6 +342,7 @@ export function initImprovementModal({ apiBase = "http://127.0.0.1:8000", domain
         description: suggestion,
         debt_type: "dashboard-improvement",
         severity: "low",
+        status: "submitted",
         location,
       };