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/answer.py

@@ -8,13 +8,20 @@ from typing import TYPE_CHECKING
 from python.ebook_search.llm_interface import request_chat_completion
 
 if TYPE_CHECKING:
+    import httpx
+
     from python.ebook_search.config import EbookSearchConfig
     from python.ebook_search.search import SearchResult
 
 logger = logging.getLogger(__name__)
 
 
-def answer_query(query: str, results: list[SearchResult], config: EbookSearchConfig) -> str:
+async def answer_query(
+    client: httpx.AsyncClient,
+    query: str,
+    results: list[SearchResult],
+    config: EbookSearchConfig,
+) -> str:
     """Answer a question using only retrieved chunks."""
     if not config.answer_enabled:
         logger.info("ebook_answer_skipped_disabled")
@@ -35,7 +42,8 @@ def answer_query(query: str, results: list[SearchResult], config: EbookSearchCon
         f"[{index}] {result.source_title}{' - ' + result.chapter_title if result.chapter_title else ''}\n{result.text}"
         for index, result in enumerate(results, start=1)
     )
-    content = request_chat_completion(
+    content = await request_chat_completion(
+        client,
         config,
         [
             {