Practice area separation: multi-tenant axis across DB, RAG, and UI

Adds two orthogonal columns — practice_area (top-level legal domain:
appeals_committee / national_insurance / labor_law) and appeal_subtype
(building_permit / betterment_levy / compensation_197) — denormalized
into cases, documents, document_chunks, decisions, and style_corpus so
vector searches can filter without JOINs.

Why: the system handles two unrelated sub-domains under the same
appeals committee (1xxx building permits and 8xxx/9xxx betterment/197),
with different rules and writing style. Without a separation axis,
search_similar() and the block-writer's precedent lookup were free to
surface betterment-levy paragraphs while drafting a building-permit
decision — a real risk of cross-domain contamination. The same axis
also lets future domains (national insurance, labor law) coexist
without separate schemas.

Schema (V4 migration in db.py):
- ALTER ... ADD COLUMN IF NOT EXISTS on all five tables + composite
  indexes (practice_area first).
- Idempotent backfill: case_number ~ '^1' → building_permit, '^8' →
  betterment_levy, '^9' → compensation_197; propagated to documents,
  chunks, and decisions via case_id; training-corpus rows (case_id NULL)
  default to appeals_committee.

Code:
- New services/practice_area.py with derive_subtype, validate, and
  is_override + enum constants.
- db.create_case / create_document / store_chunks / create_decision
  inherit practice_area from the parent case (or take an explicit
  override for the case_id=None training corpus).
- db.search_similar and search_similar_paragraphs accept practice_area
  + appeal_subtype filters using the denormalized columns.
- tools/search.py auto-resolves the filter from case_number when given.
- block_writer._build_precedents_context now passes the active case's
  practice_area to search_similar_paragraphs — closes the contamination
  hole for the discussion-block precedent fetch.
- tools/cases.case_create auto-derives subtype from case_number; an
  explicit override that disagrees writes a case_subtype_override entry
  to audit_log so we can spot bad classifications later.
- tools/documents.document_upload_training tags new training material
  with practice_area + subtype end-to-end (corpus, document, chunks).

UI (web/static/index.html + web/app.py):
- New-case wizard gets a practice_area dropdown (others disabled until
  national_insurance / labor_law arrive) and an appeal_subtype dropdown
  with JS auto-fill from the case-number prefix; manual edits stick.
- Case header shows a blue badge with practice_area · subtype.
- CaseCreateRequest plumbs both fields through to cases_tools.case_create.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/app.py

@@ -1069,6 +1069,8 @@ class CaseCreateRequest(BaseModel):
     hearing_date: str = ""
     notes: str = ""
     expected_outcome: str = ""
+    practice_area: str = "appeals_committee"
+    appeal_subtype: str = ""
 
 
 class CaseUpdateRequest(BaseModel):
@@ -1097,6 +1099,8 @@ async def api_case_create(req: CaseCreateRequest):
         hearing_date=req.hearing_date,
         notes=req.notes,
         expected_outcome=req.expected_outcome,
+        practice_area=req.practice_area,
+        appeal_subtype=req.appeal_subtype,
     )
     return json.loads(result)