feat(learning): grow style-exemplars from every final (P1 #5)

The style-exemplar corpus (channel B — the writer's block-level retrieval of
Dafna's real prose) was FROZEN at the one-time seed backfill: new finals were
enrolled into style_corpus but never broken into exemplars, so the richest style
channel never grew (8137/8126/8174 had 0 exemplars). The writer kept retrieving
only March–April seed paragraphs no matter how many finals were signed.

Extract the per-decision exemplar logic (section→paragraph→Voyage-embed→replace)
into a shared service `legal_mcp.services.style_exemplars.extract_and_store` —
the SINGLE implementation now used by BOTH the one-time backfill and the live
enrollment path (G2; no parallel extractor). `_enroll_final_in_library` calls it
on every final upload (source='internal_committee', the same source the writer's
search_style_exemplars reads). Voyage embeds over REST → container-safe;
best-effort, surfaced in the upload response, never fails the upload.

Effect: every signed final now grows the exemplar corpus, so the writer's
block-level style retrieval improves with each decision — the core "learn from
every decision" fix for channel B. Path A (style_distance_history) will track
whether the larger exemplar pool reduces style-distance over time.

Invariants: G2 (one extraction path shared by backfill + enroll), INV-LRN5
(style/structure prose only — substance routes elsewhere), INV-LRN4 (the
draft↔final loop now feeds the exemplar channel, not just the lesson channel).

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

web/app.py

@@ -3769,6 +3769,22 @@ async def _enroll_final_in_library(
         logger.warning("inline metadata extraction failed for %s: %s", case_number, e)
         out["metadata_error"] = str(e)
 
+    # Grow the style-exemplar corpus (channel B): break this final into block-level
+    # paragraphs the writer retrieves (07-learning §0.2). Before this, exemplars were
+    # frozen at the one-time seed backfill — new finals never got exemplarized, so the
+    # richest style channel never grew. Same extraction the backfill uses (G2). Voyage
+    # embeds over REST → container-safe. Best-effort; surfaced, never fails the upload.
+    try:
+        from legal_mcp.services import style_exemplars as _sx
+        n_ex = await _sx.extract_and_store(
+            decision_number=case_number, source="internal_committee",
+            full_text=final_text, practice_area=case.get("practice_area", ""),
+        )
+        out["exemplars"] = n_ex
+    except Exception as e:
+        logger.warning("style-exemplar extraction failed for %s: %s", case_number, e)
+        out["exemplars_error"] = str(e)
+
     # The precedents this decision cites → link to the library; flag the ones not found.
     try:
         await cit_tools.extract_internal_citations(case_law_id=case_law_id, limit=0)