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/dependencies.py

@@ -4,8 +4,9 @@ from __future__ import annotations
 
 from typing import Annotated
 
+import httpx
 from fastapi import Depends, Request
-from sqlalchemy.engine import Engine
+from sqlalchemy.ext.asyncio import AsyncEngine
 
 from python.ebook_search.config import EbookSearchConfig
 
@@ -15,10 +16,16 @@ def get_config(request: Request) -> EbookSearchConfig:
     return request.app.state.config
 
 
-def get_engine(request: Request) -> Engine:
+def get_engine(request: Request) -> AsyncEngine:
     """Get the database engine from app state."""
     return request.app.state.engine
 
 
+def get_http_client(request: Request) -> httpx.AsyncClient:
+    """Get the shared LLM HTTP client from app state."""
+    return request.app.state.http_client
+
+
 AppConfig = Annotated[EbookSearchConfig, Depends(get_config)]
-AppEngine = Annotated[Engine, Depends(get_engine)]
+AppEngine = Annotated[AsyncEngine, Depends(get_engine)]
+AppHttpClient = Annotated[httpx.AsyncClient, Depends(get_http_client)]