feat(digests): case-contextual digest radar — surface unlinked digests as chair leads (X12)

A digest pointing at a ruling we don't hold yet ("unlinked") was captured globally
(missing_precedents inbox) but never surfaced IN THE CONTEXT of the case being
decided — so a relevant ruling known only via a digest could fall through the cracks
at the moment it matters. Adds the case-contextual radar:

- db.search_digests_semantic: new `linked_only` filter (False = unlinked-only target set).
- digest_library.case_digest_radar(case_number): builds the case topic from title +
  appeal_subtype + the analyst's claims, embeds once, matches against UNLINKED digests,
  and returns leads enriched with the underlying ruling's gap status + suggested action
  (new_lead / gap_open / fetched / available_link).
- MCP tool `digest_radar` + endpoint GET /api/cases/{n}/digest-radar (for the agent and
  the future case-page lead).

INV-DIG1 preserved: radar only — every lead points at the underlying RULING (fetch /
upload / link), never cites the digest. Read-only.

Validated on 8124-09-24 (היטל השבחה): 5 on-topic unlinked digests, scores 0.64–0.72.
The visible case-page panel is a separate UI change → goes through the design gate.

Invariants: G2 (reuses the one digest semantic-search + gap/citation resolvers),
INV-DIG1 (no digest citation), INV-DIG3 (gap surfacing). No schema change.

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

mcp-server/src/legal_mcp/server.py

@@ -418,6 +418,12 @@ async def digest_process_pending(limit: int = 20) -> str:
     return await digest_tools.digest_process_pending(_clamp_limit(limit))
 
 
+@mcp.tool()
+async def digest_radar(case_number: str, limit: int = 5, min_score: float = 0.45) -> str:
+    """רדאר-יומונים הקשרי-לתיק (X12) — מחזיר יומונים לא-מקושרים (פס"ד שעוד אין לנו) שהנושא שלהם קרוב סמנטית לתיק, כדי שלא ייפול פס"ד רלוונטי בזמן ההכרעה. כל ליד מצביע על מראה-המקום של הפס"ד המקורי + סטטוס-הפער + פעולה מוצעת. ⚠️ radar בלבד — היומון אינו מצוטט (INV-DIG1)."""
+    return await digest_tools.digest_radar(case_number, _clamp_limit(limit), min_score)
+
+
 @mcp.tool()
 async def halacha_review(
     halacha_id: str,