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/protected_phrases/models.py

@@ -188,6 +188,29 @@ class PhraseCandidateGenerationResult:
     candidate_phrases: int
 
 
+@dataclass(frozen=True, slots=True)
+class CorpusPhraseStats:
+    """Corpus-wide candidate and protected phrase counts for the admin page.
+
+    Attributes:
+        total_books (int): Indexed books in the corpus.
+        books_with_candidates (int): Books that have candidate phrases generated.
+        books_fully_judged (int): Books with candidates where every candidate has been judged.
+        candidate_phrases (int): Candidate phrases stored across all books.
+        judged_candidates (int): Candidate phrases that have been LLM judged.
+        unjudged_candidates (int): Candidate phrases still waiting for judgment.
+        protected_phrases (int): Protected phrases promoted across all books.
+    """
+
+    total_books: int
+    books_with_candidates: int
+    books_fully_judged: int
+    candidate_phrases: int
+    judged_candidates: int
+    unjudged_candidates: int
+    protected_phrases: int
+
+
 @dataclass(frozen=True, slots=True)
 class PhraseJudgmentBackfillResult:
     """Summary of LLM judging for stored candidate phrases.