feat(ebook): migrate to async DB/HTTP and parallelize phrase pipeline

Convert the ebook-search web app to async end to end and add concurrency
to the protected-phrase extraction and judging pipeline so large books no
longer block the event loop or the UI.

ORM / infra:
- Add get_async_postgres_engine and factor shared URL/connect_args building
  into build_postgres_url (reused by the sync and async engine builders)
- Add async FastAPI session helpers (get_async_db, AsyncDbSession) with
  expire_on_commit=False to avoid implicit IO under asyncio

App:
- Use AsyncEngine/AsyncSession throughout routes, search, ingest, embeddings,
  answer, rerank and LLM calls; convert handlers to async
- Share a single httpx.AsyncClient in app state for LLM requests; size the
  connection pool for concurrent phrase-judging workers
- Add judge_tasks: run per-book judging as tracked background tasks so a
  book already being judged isn't double-queued

Protected phrases:
- Add a process pool (pool.py) and worker-count config
  (extraction/judge book/phrase workers) to parallelize candidate generation
  and judging
- Split admin actions into all/missing variants for generation and judging

Config:
- Add protected_phrase_extraction_workers, phrase_judge_book_workers,
  phrase_judge_phrase_workers

python/ebook_search/api/bm25_tasks.py

@@ -1,18 +1,22 @@
-"""Background BM25 refresh tasks for the web app."""
+"""Background BM25 refresh tasks for the web app.
+
+The refresh is scheduled on the event loop instead of a thread because the async psycopg
+driver only works from the loop; a bare thread cannot open a session on the async engine.
+"""
 
 from __future__ import annotations
 
+import asyncio
 import logging
-from threading import Timer
 from typing import TYPE_CHECKING
 
-from sqlalchemy.orm import Session
+from sqlalchemy.ext.asyncio import AsyncSession
 
 from python.ebook_search.bm25_corpus import load_bm25_corpus, refresh_bm25_corpus
 
 if TYPE_CHECKING:
     from fastapi import FastAPI
-    from sqlalchemy.engine import Engine
+    from sqlalchemy.ext.asyncio import AsyncEngine
 
     from python.ebook_search.config import EbookSearchConfig
 
@@ -20,15 +24,18 @@ logger = logging.getLogger(__name__)
 
 
 def schedule_bm25_refresh(app: FastAPI) -> None:
-    """Schedule a delayed BM25 corpus refresh, replacing any pending refresh."""
-    existing_timer = getattr(app.state, "bm25_refresh_timer", None)
-    if existing_timer is not None:
-        existing_timer.cancel()
+    """Schedule a delayed BM25 corpus refresh, replacing any pending refresh.
 
-    timer = Timer(app.state.config.bm25_refresh_delay_seconds, refresh_bm25_for_app, args=(app,))
-    timer.daemon = True
-    timer.start()
-    app.state.bm25_refresh_timer = timer
+    Only called from route handlers, so a running event loop is guaranteed.
+    """
+    cancel_bm25_refresh(app)
+
+    loop = asyncio.get_running_loop()
+
+    def start_refresh() -> None:
+        app.state.bm25_refresh_task = loop.create_task(refresh_bm25_for_app(app))
+
+    app.state.bm25_refresh_timer = loop.call_later(app.state.config.bm25_refresh_delay_seconds, start_refresh)
     logger.info(
         "ebook_bm25_refresh_scheduled delay_seconds=%s",
         app.state.config.bm25_refresh_delay_seconds,
@@ -36,25 +43,31 @@ def schedule_bm25_refresh(app: FastAPI) -> None:
 
 
 def cancel_bm25_refresh(app: FastAPI) -> None:
-    """Cancel any pending BM25 corpus refresh."""
+    """Cancel any pending BM25 corpus refresh timer and in-flight refresh task."""
     existing_timer = getattr(app.state, "bm25_refresh_timer", None)
     if existing_timer is not None:
         existing_timer.cancel()
         app.state.bm25_refresh_timer = None
         logger.info("ebook_bm25_refresh_cancelled")
 
+    existing_task = getattr(app.state, "bm25_refresh_task", None)
+    if existing_task is not None:
+        if not existing_task.done():
+            existing_task.cancel()
+        app.state.bm25_refresh_task = None
 
-def refresh_bm25_for_app(app: FastAPI) -> None:
+
+async def refresh_bm25_for_app(app: FastAPI) -> None:
     """Refresh the BM25 corpus using the app engine and config."""
     try:
-        refresh_bm25_for_engine(app.state.engine, app.state.config)
+        await refresh_bm25_for_engine(app.state.engine, app.state.config)
     except Exception:
         logger.exception("ebook_bm25_refresh_failed")
 
 
-def refresh_bm25_for_engine(engine: Engine, config: EbookSearchConfig) -> None:
-    """Refresh the BM25 corpus using a SQLAlchemy engine."""
-    with Session(engine) as session:
-        refresh_bm25_corpus(session, config)
+async def refresh_bm25_for_engine(engine: AsyncEngine, config: EbookSearchConfig) -> None:
+    """Refresh the BM25 corpus using an async SQLAlchemy engine."""
+    async with AsyncSession(engine) as session:
+        await refresh_bm25_corpus(session, config)
     load_bm25_corpus.cache_clear()
     logger.info("ebook_bm25_corpus_cache_cleared_after_refresh")