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/templates/book_detail.html

@@ -30,6 +30,15 @@
         >
             <button type="submit">Recalculate phrases</button>
         </form>
+        <form
+            method="post"
+            action="/books/{{ source.id }}/judge-phrases"
+            onsubmit="return confirm('Judge candidate phrases for this book with the LLM?');"
+        >
+            <button type="submit"{% if judging_in_progress %} disabled{% endif %}>
+                {% if judging_in_progress %}Judging&hellip;{% else %}Judge phrases{% endif %}
+            </button>
+        </form>
 
         <section>
             <h2>Candidate n-grams</h2>