feat(precedent-library): add district and chair_name to edit form

Fields existed in DB and Precedent type but were missing from:
- PrecedentUpdateRequest (backend model)
- update_case_law allowed set (db layer)
- PrecedentPatch (frontend type)
- precedent-edit-sheet form state, inputs, and patch payload

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

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

@@ -1985,7 +1985,7 @@ async def update_case_law(case_law_id: UUID, **fields) -> dict | None:
     allowed = {
         "case_number", "case_name", "court", "date", "practice_area", "appeal_subtype",
         "subject_tags", "summary", "headnote", "key_quote", "source_url",
-        "source_type", "precedent_level", "is_binding",
+        "source_type", "precedent_level", "is_binding", "district", "chair_name",
     }
     updates = {k: v for k, v in fields.items() if k in allowed}
     if not updates: