feat(citation-verify): backend for the "אימות פסיקה" panel — per-argument support + verify gate (#154)

Backend half of the citation-verification tab (frontend follows in a separate PR):

- Schema V44: case_precedents gains argument_id (the legal argument it supports),
  case_law_id (the corpus ruling — for cited_by + dedup), verified (the INV-AH gate
  the writer respects) + verified_at. All nullable; chair_note already existed.
- db: create_case_precedent(+argument_id/case_law_id/verified),
  set_case_precedent_verified(id, verified, chair_note), list_case_precedents(+cols).
- services/case_citation_verification.build_view(case): per legal_argument →
  in-corpus suggestions (search_library per issue) each with the cited_by authority
  breakdown (db.citation_authority, X11) + merged attach/verify state + per-issue
  radar (case_digest_radar grouped by matched issue). Pure read/assembly; reuses the
  one corpus search + one authority query + one radar (G2).
- endpoints: GET /api/cases/{n}/citation-verification (the view) and
  POST .../verify (upsert verify/un-verify + chair note).

Validated on 1044-03-26: 14 arguments, all with support; 317/10 surfaces with
אומץ×11/אובחן×1; radar leads attributed per issue. Auto-approval untouched (chair
gate, INV-G10). Verify defaults to false — nothing is authoritative until the chair
marks it.

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

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

@@ -1731,6 +1731,23 @@ UPDATE case_law SET district = 'חיפה'
 """
 
 
+SCHEMA_V44_SQL = """
+-- Citation-verification panel (#154): the chair verifies each corpus precedent
+-- against the specific legal ARGUMENT before the writer may cite it (INV-AH gate).
+-- case_precedents gains: the argument it supports, the resolved case_law row
+-- (for the cited_by authority signal + dedup), a verified flag (the gate), and a
+-- verification timestamp. chair_note already exists. All nullable so legacy
+-- section-scoped rows stay valid.
+ALTER TABLE case_precedents ADD COLUMN IF NOT EXISTS argument_id UUID
+    REFERENCES legal_arguments(id) ON DELETE SET NULL;
+ALTER TABLE case_precedents ADD COLUMN IF NOT EXISTS case_law_id UUID
+    REFERENCES case_law(id) ON DELETE SET NULL;
+ALTER TABLE case_precedents ADD COLUMN IF NOT EXISTS verified BOOLEAN DEFAULT false;
+ALTER TABLE case_precedents ADD COLUMN IF NOT EXISTS verified_at TIMESTAMPTZ;
+CREATE INDEX IF NOT EXISTS idx_case_precedents_argument ON case_precedents(argument_id);
+"""
+
+
 # Stable, arbitrary key for the session-level advisory lock that serialises
 # schema DDL across processes. Every short-lived process (cron drains, services)
 # re-runs the idempotent migrations on startup; without this lock two processes
@@ -1796,6 +1813,7 @@ async def _apply_schema_ddl(conn: asyncpg.Connection) -> None:
         await conn.execute(SCHEMA_V41_SQL)
         await conn.execute(SCHEMA_V42_SQL)
         await conn.execute(SCHEMA_V43_SQL)
+        await conn.execute(SCHEMA_V44_SQL)
 
 
 async def init_schema() -> None:
@@ -3331,28 +3349,58 @@ async def create_case_precedent(
     chair_note: str = "",
     pdf_document_id: UUID | None = None,
     practice_area: str | None = None,
+    argument_id: UUID | None = None,
+    case_law_id: UUID | None = None,
+    verified: bool = False,
 ) -> dict:
-    """Insert a new precedent attached to a case."""
+    """Insert a new precedent attached to a case.
+
+    ``argument_id``/``case_law_id``/``verified`` (X11 #154) link the attachment to
+    the specific legal argument it supports and the corpus ruling, and mark whether
+    the chair verified it (the INV-AH gate the writer respects)."""
     pool = await get_pool()
     row = await pool.fetchrow(
         """
         INSERT INTO case_precedents
-            (case_id, section_id, quote, citation, chair_note, pdf_document_id, practice_area)
-        VALUES ($1, $2, $3, $4, $5, $6, $7)
+            (case_id, section_id, quote, citation, chair_note, pdf_document_id,
+             practice_area, argument_id, case_law_id, verified, verified_at)
+        VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10,
+                CASE WHEN $10 THEN now() ELSE NULL END)
         RETURNING *
         """,
-        case_id, section_id, quote, citation, chair_note, pdf_document_id, practice_area,
+        case_id, section_id, quote, citation, chair_note, pdf_document_id,
+        practice_area, argument_id, case_law_id, verified,
     )
     return dict(row)
 
 
+async def set_case_precedent_verified(
+    precedent_id: UUID, verified: bool, chair_note: str | None = None,
+) -> dict | None:
+    """Toggle the chair-verification gate on an attached precedent (#154), optionally
+    updating the chair note. Sets verified_at when verifying, clears it when un-verifying."""
+    pool = await get_pool()
+    sets = ["verified = $2", "verified_at = CASE WHEN $2 THEN now() ELSE NULL END",
+            "updated_at = now()"]
+    params: list = [precedent_id, verified]
+    if chair_note is not None:
+        sets.append(f"chair_note = ${len(params) + 1}")
+        params.append(chair_note)
+    row = await pool.fetchrow(
+        f"UPDATE case_precedents SET {', '.join(sets)} WHERE id = $1 RETURNING *",
+        *params,
+    )
+    return dict(row) if row else None
+
+
 async def list_case_precedents(case_id: UUID) -> list[dict]:
     """List all precedents attached to a case, ordered by section then creation time."""
     pool = await get_pool()
     rows = await pool.fetch(
         """
         SELECT id, case_id, section_id, quote, citation, chair_note,
-               pdf_document_id, practice_area, created_at, updated_at
+               pdf_document_id, practice_area, argument_id, case_law_id,
+               verified, verified_at, created_at, updated_at
         FROM case_precedents
         WHERE case_id = $1
         ORDER BY section_id NULLS LAST, created_at