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/routes/search.py

@@ -14,6 +14,7 @@ from python.ebook_search.answer import answer_query
 from python.ebook_search.api.dependencies import (  # noqa: TC001 FastAPI resolves these annotated dependencies at runtime
     AppConfig,
     AppEngine,
+    AppHttpClient,
 )
 from python.ebook_search.api.web import templates
 from python.ebook_search.guardrails import (
@@ -26,6 +27,8 @@ from python.ebook_search.search import SearchResponse, search_ebooks
 from python.ebook_search.timing import runtime_step_from_start
 
 if TYPE_CHECKING:
+    import httpx
+
     from python.ebook_search.config import EbookSearchConfig
 
 logger = logging.getLogger(__name__)
@@ -33,7 +36,8 @@ logger = logging.getLogger(__name__)
 router = APIRouter()
 
 
-def build_answer(
+async def build_answer(
+    client: httpx.AsyncClient,
     query: str,
     response: SearchResponse,
     config: EbookSearchConfig,
@@ -56,7 +60,7 @@ def build_answer(
         return answer, True, None
 
     try:
-        answer = answer_query(query, response.results, config)
+        answer = await answer_query(client, query, response.results, config)
     except RuntimeError as error:
         logger.warning("ebook_answer_request_failed_falling_back error=%s", error)
         return "Answer generation failed. Source chunks are still shown below.", False, None
@@ -74,18 +78,20 @@ def build_answer(
 
 
 @router.post("/search", response_class=HTMLResponse)
-def search(
+async def search(
     request: Request,
     config: AppConfig,
     engine: AppEngine,
+    client: AppHttpClient,
     query: Annotated[str, Form()],
     rerank: Annotated[str | None, Form()] = None,
     phrase_matching: Annotated[str | None, Form()] = None,
 ) -> HTMLResponse:
     """Run a search and render HTMX results."""
     try:
-        response = search_ebooks(
+        response = await search_ebooks(
             engine,
+            client,
             query,
             config,
             rerank=rerank == "true",
@@ -96,7 +102,7 @@ def search(
         return templates.TemplateResponse(request, "partials/error.html", {"message": str(error)}, status_code=500)
 
     answer_start = perf_counter()
-    answer, low_confidence, citation_report = build_answer(query, response, config)
+    answer, low_confidence, citation_report = await build_answer(client, query, response, config)
     answer_step_name = "Answer generation" if config.answer_enabled else "Answer skipped"
     response = replace(
         response,