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/timing.py

@@ -7,7 +7,7 @@ from time import perf_counter
 from typing import TYPE_CHECKING
 
 if TYPE_CHECKING:
-    from collections.abc import Callable
+    from collections.abc import Awaitable, Callable
 
 
 @dataclass(frozen=True)
@@ -34,3 +34,10 @@ def timed_result[T, **P](
     start_seconds = perf_counter()
     result = operation(*args, **kwargs)
     return result, runtime_step_from_start(name, start_seconds)
+
+
+async def async_timed_result[T](name: str, awaitable: Awaitable[T]) -> tuple[T, RuntimeStep]:
+    """Await an operation and return its result plus elapsed runtime."""
+    start_seconds = perf_counter()
+    result = await awaitable
+    return result, runtime_step_from_start(name, start_seconds)