feat(status): single source of truth for the case-status model + canonicalize intermediates

Fixes the case where the H1 chip, the pipeline stepper and the manual-changer
dropdown disagreed (e.g. 1043-02-26 on analyst_verified): the status sat between
the canonical 10 and a legacy bucket, so each surface read a different list.

Root fix — ONE authoritative status model + canonicalize the real intermediate
states (chair-approved):

- New mcp-server/.../case_status_model.py — the single registry: ordered
  StatusDef list (key/label/description/phase/selectable/terminal/on_enter),
  the 5 phases, STATUS_ORDER, phase_of/label_of, and a drift assertion that the
  CaseStatus enum matches it. `on_enter` is the seam for a status to *do*
  something on entry, declared next to its definition (no dispatcher wired yet).
- models.CaseStatus enum: analyst_verified + research_complete promoted to
  first-class canonical statuses (the agents set them — they belong in the
  model, not a legacy fallback).
- tools/cases.py: the forward-only STATUS_ORDER guard now derives from the
  registry instead of an inline list.
- GET /api/status-model exposes the model so the frontend mirror can be
  generated against it.
- web-ui case-status.ts mirrors it: the two intermediates moved from the
  legacy map into CASE_STATUSES / PHASES(thinking) / STATUS_LABELS /
  STATUS_DESCRIPTIONS; status-badge icons+tones extended. So the chip (status),
  stepper (its phase) and dropdown (now includes it) all agree.

Invariants: G2 — collapses the scattered status definitions (enum, inline
STATUS_ORDER, two frontend label maps) onto one backend authority + a documented
frontend mirror; no parallel status list remains. Agent-prompt alignment +
backfill follow in a separate change. py_compile + tsc + eslint clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/app.py

@@ -1828,6 +1828,16 @@ async def health():
     return {"status": "ok"}
 
 
+@app.get("/api/status-model")
+async def api_status_model():
+    """Canonical case-status model — ordered statuses (key/label/description/
+    phase/selectable/terminal/on_enter) + the 5 phases. Single source of truth
+    (legal_mcp/case_status_model.py); the frontend case-status.ts mirrors it."""
+    from legal_mcp.case_status_model import to_dict
+
+    return to_dict()
+
+
 @app.get("/api/cases")
 async def list_cases(
     detail: bool = False,