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/main.py

@@ -6,11 +6,12 @@ import logging
 from contextlib import asynccontextmanager
 from typing import TYPE_CHECKING, Annotated
 
+import httpx
 import typer
 import uvicorn
 from fastapi import FastAPI
 from fastapi.staticfiles import StaticFiles
-from sqlalchemy.orm import Session
+from sqlalchemy.ext.asyncio import AsyncSession
 
 from python.common import configure_logger
 from python.ebook_search.api.bm25_tasks import cancel_bm25_refresh
@@ -18,8 +19,9 @@ from python.ebook_search.api.routes import admin_router, health_router, page_rou
 from python.ebook_search.api.web import STATIC_DIR
 from python.ebook_search.bm25_corpus import ensure_bm25_corpus
 from python.ebook_search.config import load_config
+from python.ebook_search.protected_phrases.pool import shutdown_extraction_pool
 from python.fastapi_tools import ZstdMiddleware
-from python.orm.common import get_postgres_engine
+from python.orm.common import get_async_postgres_engine
 
 if TYPE_CHECKING:
     from collections.abc import AsyncIterator
@@ -48,15 +50,24 @@ async def lifespan(app: FastAPI) -> AsyncIterator[None]:
     )
     if not config.library_paths:
         logger.warning("ebook_search_no_library_paths_configured")
-    app.state.engine = get_postgres_engine(name="RICHIE", vector_engine=True)
-    with Session(app.state.engine) as session:
-        ensure_bm25_corpus(session, config)
+    # Concurrent phrase judging opens one session per book worker on this engine, so size the pool
+    # to cover those plus headroom for ordinary web requests.
+    app.state.engine = get_async_postgres_engine(
+        name="RICHIE",
+        vector_engine=True,
+        pool_size=config.phrase_judge_book_workers + 10,
+    )
+    app.state.http_client = httpx.AsyncClient()
+    async with AsyncSession(app.state.engine, expire_on_commit=False) as session:
+        await ensure_bm25_corpus(session, config)
     try:
         yield
     finally:
         logger.info("ebook_search_shutdown")
         cancel_bm25_refresh(app)
-        app.state.engine.dispose()
+        shutdown_extraction_pool()
+        await app.state.http_client.aclose()
+        await app.state.engine.dispose()
 
 
 def create_app() -> FastAPI: