fix(precedents): Anthropic SDK fallback, format() crash, UI refresh

Three fixes to the precedent library after the first end-to-end test on
403-17 surfaced runtime issues:

1. Anthropic SDK fallback in claude_session. The legal-ai Docker container
   does not ship the `claude` CLI, so every halacha and metadata extraction
   was failing with "Claude CLI not found." Module now tries the CLI first
   (zero-cost local path) and falls back to the Anthropic SDK with
   ANTHROPIC_API_KEY when the binary is absent. Default model is
   claude-sonnet-4-6, overridable via CLAUDE_SDK_MODEL env. The system
   message gets cache_control: ephemeral so multi-chunk runs reuse the
   cached instruction prefix at ~10% read cost. Adds `anthropic` to
   pyproject deps.

2. precedent_metadata_extractor crashed with KeyError because the JSON
   example inside the prompt template contained literal { } characters
   that str.format() interpreted as placeholders. Switched to f-string
   concatenation; the prompt template no longer needs format() at all.

3. Library list query stays stale after upload because the upload
   mutation's onSuccess fires when the POST returns task_id, not when
   SSE reports completion. Added a second invalidate inside the SSE
   watcher in PrecedentUploadSheet so the new row appears with up-to-date
   chunk and halachot counts the moment processing finishes.

Halacha and metadata extractors now route the long static prompt through
the new `system=` parameter so the SDK path actually caches it; the CLI
path concatenates and behaves as before.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -279,8 +279,9 @@ async def _extract_chunk(
         else HALACHA_EXTRACTION_PROMPT_PERSUASIVE
     )
     chunk_label = f" (חלק {chunk_index + 1}/{chunk_total})" if chunk_total > 1 else ""
-    prompt = (
-        f"{base_prompt}\n\n"
+    # Pass the static instruction prompt as `system` so the SDK path can cache
+    # it (5-min ephemeral). Only the per-chunk content varies via `prompt`.
+    user_msg = (
         f"## הקלט\n"
         f"סוג קטע: {section_type}\n"
         f"{context}{chunk_label}\n\n"
@@ -289,7 +290,7 @@ async def _extract_chunk(
     last_err: Exception | None = None
     for attempt in range(CHUNK_RETRY_ATTEMPTS + 1):
         try:
-            result = await claude_session.query_json(prompt)
+            result = await claude_session.query_json(user_msg, system=base_prompt)
         except Exception as e:
             last_err = e
             logger.warning(