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>

mcp-server/src/legal_mcp/services/practice_area.py

@@ -0,0 +1,96 @@
+"""Practice area + appeal subtype: derivation, validation, constants.
+
+Two orthogonal axes used to separate legal domains across the system:
+
+  practice_area  — top-level domain (multi-tenant axis). Examples:
+                   appeals_committee, national_insurance, labor_law.
+  appeal_subtype — refines within a domain. For appeals_committee:
+                   building_permit (1xxx), betterment_levy (8xxx),
+                   compensation_197 (9xxx), unknown.
+
+Both columns are denormalized into documents/chunks/decisions/style_corpus
+so vector searches can filter cheaply.
+"""
+
+from __future__ import annotations
+
+import re
+
+# ── Enums ──────────────────────────────────────────────────────────
+
+PRACTICE_AREAS: set[str] = {
+    "appeals_committee",
+    "national_insurance",
+    "labor_law",
+}
+
+APPEALS_COMMITTEE_SUBTYPES: set[str] = {
+    "building_permit",
+    "betterment_levy",
+    "compensation_197",
+    "unknown",
+}
+
+DEFAULT_PRACTICE_AREA = "appeals_committee"
+
+# Subtypes per practice_area (extend when adding domains)
+SUBTYPES_BY_AREA: dict[str, set[str]] = {
+    "appeals_committee": APPEALS_COMMITTEE_SUBTYPES,
+    "national_insurance": {"unknown"},
+    "labor_law": {"unknown"},
+}
+
+
+# ── Derivation ─────────────────────────────────────────────────────
+
+_FIRST_DIGIT = re.compile(r"^\s*(\d)")
+
+_APPEALS_COMMITTEE_DIGIT_TO_SUBTYPE = {
+    "1": "building_permit",
+    "8": "betterment_levy",
+    "9": "compensation_197",
+}
+
+
+def derive_subtype(case_number: str, practice_area: str = DEFAULT_PRACTICE_AREA) -> str:
+    """Infer the appeal_subtype from case_number.
+
+    For appeals_committee, the convention is:
+      1xxx → building_permit, 8xxx → betterment_levy, 9xxx → compensation_197.
+
+    For other practice areas there is no public numbering convention yet,
+    so we return 'unknown' until a real rule is defined.
+    """
+    if practice_area != "appeals_committee":
+        return "unknown"
+    m = _FIRST_DIGIT.match(case_number or "")
+    if not m:
+        return "unknown"
+    return _APPEALS_COMMITTEE_DIGIT_TO_SUBTYPE.get(m.group(1), "unknown")
+
+
+# ── Validation ─────────────────────────────────────────────────────
+
+
+def validate(practice_area: str, appeal_subtype: str | None) -> None:
+    """Raise ValueError on unknown values. appeal_subtype=None is allowed."""
+    if practice_area not in PRACTICE_AREAS:
+        raise ValueError(
+            f"unknown practice_area: {practice_area!r}. "
+            f"expected one of {sorted(PRACTICE_AREAS)}"
+        )
+    if appeal_subtype is None:
+        return
+    allowed = SUBTYPES_BY_AREA.get(practice_area, {"unknown"})
+    if appeal_subtype not in allowed:
+        raise ValueError(
+            f"unknown appeal_subtype {appeal_subtype!r} for practice_area "
+            f"{practice_area!r}. expected one of {sorted(allowed)}"
+        )
+
+
+def is_override(case_number: str, practice_area: str, appeal_subtype: str) -> bool:
+    """True iff the user-supplied subtype disagrees with what derive_subtype
+    would have produced (and the derived value is not 'unknown')."""
+    derived = derive_subtype(case_number, practice_area)
+    return derived != "unknown" and derived != appeal_subtype