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/admin.html

@@ -8,37 +8,25 @@ head %}
   <form hx-post="/admin/scan" hx-target="#admin-status" hx-swap="innerHTML">
     <button type="submit">Scan</button>
   </form>
-  <form
-    hx-post="/admin/generate-ngrams"
-    hx-target="#admin-status"
-    hx-swap="innerHTML"
-  >
-    <button type="submit">Generate n-grams</button>
-  </form>
-  <form
-    hx-post="/admin/judge-ngrams"
-    hx-target="#admin-status"
-    hx-swap="innerHTML"
-  >
-    <button type="submit">Judge n-grams</button>
-  </form>
-  <form
-    hx-post="/admin/embed-missing"
-    hx-target="#admin-status"
-    hx-swap="innerHTML"
-  >
-    <button type="submit">Embed</button>
-  </form>
-  <form
-    hx-post="/admin/embed-all"
-    hx-target="#admin-status"
-    hx-swap="innerHTML"
-  >
-    <button type="submit">Embed all</button>
-  </form>
 </section>
 <section>
   <h2>Embeddings</h2>
+  <section class="actions">
+    <form
+      hx-post="/admin/embed-missing"
+      hx-target="#admin-status"
+      hx-swap="innerHTML"
+    >
+      <button type="submit">Embed</button>
+    </form>
+    <form
+      hx-post="/admin/embed-all"
+      hx-target="#admin-status"
+      hx-swap="innerHTML"
+    >
+      <button type="submit">Embed all</button>
+    </form>
+  </section>
   <table>
     <thead>
       <tr>
@@ -62,4 +50,61 @@ head %}
     </tbody>
   </table>
 </section>
+<section>
+  <h2>Protected phrases</h2>
+  <section class="actions">
+    <form
+      hx-post="/admin/phrases/generate-all"
+      hx-target="#admin-status"
+      hx-swap="innerHTML"
+    >
+      <button type="submit">Regenerate all phrases</button>
+    </form>
+    <form
+      hx-post="/admin/phrases/generate-missing"
+      hx-target="#admin-status"
+      hx-swap="innerHTML"
+    >
+      <button type="submit">Add missing phrases</button>
+    </form>
+    <form
+      hx-post="/admin/phrases/judge-all"
+      hx-target="#admin-status"
+      hx-swap="innerHTML"
+    >
+      <button type="submit">Judge all phrases</button>
+    </form>
+    <form
+      hx-post="/admin/phrases/judge-missing"
+      hx-target="#admin-status"
+      hx-swap="innerHTML"
+    >
+      <button type="submit">Judge missing phrases</button>
+    </form>
+  </section>
+  <table>
+    <thead>
+      <tr>
+        <th>Candidates</th>
+        <th>Judged</th>
+        <th>Unjudged</th>
+        <th>Protected</th>
+        <th>Books indexed</th>
+        <th>Books generated</th>
+        <th>Books fully judged</th>
+      </tr>
+    </thead>
+    <tbody>
+      <tr>
+        <td>{{ phrase_stats.candidate_phrases }}</td>
+        <td>{{ phrase_stats.judged_candidates }}</td>
+        <td>{{ phrase_stats.unjudged_candidates }}</td>
+        <td>{{ phrase_stats.protected_phrases }}</td>
+        <td>{{ phrase_stats.total_books }}</td>
+        <td>{{ phrase_stats.books_with_candidates }}</td>
+        <td>{{ phrase_stats.books_fully_judged }}</td>
+      </tr>
+    </tbody>
+  </table>
+</section>
 {% endblock %}