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/judge_tasks.py

@@ -0,0 +1,131 @@
+"""Background phrase-judging tasks for the web app.
+
+Judging a book sends one LLM request per candidate phrase, which can take minutes, so it must
+not run inside the request where it would block the UI. Judgments run as async FastAPI
+background tasks, awaited on the event loop after the response is sent, and are tracked per
+book in app state so a second judge request for a book that is already being judged is
+rejected instead of doubling the work.
+
+State is loop-confined: every read and mutation happens on the event loop (async route
+handlers and async background tasks) and no critical section contains an ``await``, so each
+mutation is atomic per loop iteration and no locking is needed.
+"""
+
+from __future__ import annotations
+
+import logging
+from dataclasses import dataclass, field
+from typing import TYPE_CHECKING
+
+from python.ebook_search.protected_phrases.judge_ngrams import judge_candidate_phrases_for_books
+
+if TYPE_CHECKING:
+    from fastapi import BackgroundTasks, FastAPI
+
+logger = logging.getLogger(__name__)
+
+
+@dataclass
+class JudgeTaskState:
+    """Running book judgments and last outcome messages, keyed by book id."""
+
+    running_book_ids: set[int] = field(default_factory=set)
+    outcome_messages: dict[int, str] = field(default_factory=dict)
+
+
+def get_judge_task_state(app: FastAPI) -> JudgeTaskState:
+    """Return the app's judge task state, creating it on first use.
+
+    Args:
+        app (FastAPI): App whose state holds the judge task registry.
+
+    Returns:
+        JudgeTaskState: The shared judge task state for this app.
+    """
+    state = getattr(app.state, "judge_tasks", None)
+    if state is None:
+        state = JudgeTaskState()
+        app.state.judge_tasks = state
+    return state
+
+
+def start_book_phrase_judgment(app: FastAPI, background_tasks: BackgroundTasks, source_id: int) -> bool:
+    """Queue judging of one book's candidate phrases as a FastAPI background task.
+
+    The book is claimed before the response returns, so a repeated judge request cannot queue
+    a second run while one is pending or running.
+
+    Args:
+        app (FastAPI): App supplying the engine, config, and judge task state.
+        background_tasks (BackgroundTasks): Request's background tasks to queue the judgment on.
+        source_id (int): Book to judge candidates for.
+
+    Returns:
+        bool: True when a judgment was queued, False when one is already running for this book.
+    """
+    state = get_judge_task_state(app)
+    if source_id in state.running_book_ids:
+        logger.info("ebook_book_phrase_judgment_already_running source_id=%s", source_id)
+        return False
+    state.running_book_ids.add(source_id)
+    state.outcome_messages.pop(source_id, None)
+    background_tasks.add_task(judge_book_phrases_for_app, app, source_id)
+    logger.info("ebook_book_phrase_judgment_queued source_id=%s", source_id)
+    return True
+
+
+async def judge_book_phrases_for_app(app: FastAPI, source_id: int) -> None:
+    """Judge one book using the app engine and config, recording the outcome message.
+
+    Args:
+        app (FastAPI): App supplying the engine, config, and judge task state.
+        source_id (int): Book to judge candidates for.
+    """
+    state = get_judge_task_state(app)
+    try:
+        result = await judge_candidate_phrases_for_books(app.state.engine, app.state.config, source_ids=[source_id])
+        logger.info(
+            "ebook_book_phrase_judgment_complete source_id=%s judged=%s protected=%s mentions=%s failed=%s",
+            source_id,
+            result.candidates_judged,
+            result.protected_phrases,
+            result.phrase_mentions,
+            result.books_failed,
+        )
+        if result.books_failed:
+            message = "Judging failed; see server logs for details"
+        else:
+            message = (
+                f"Judged {result.candidates_judged} candidates; {result.protected_phrases} protected phrases promoted"
+            )
+    except Exception:
+        logger.exception("ebook_book_phrase_judgment_task_failed source_id=%s", source_id)
+        message = "Judging failed; see server logs for details"
+    state.running_book_ids.discard(source_id)
+    state.outcome_messages[source_id] = message
+
+
+def is_judging_book(app: FastAPI, source_id: int) -> bool:
+    """Report whether a judgment is currently queued or running for one book.
+
+    Args:
+        app (FastAPI): App supplying the judge task state.
+        source_id (int): Book to check.
+
+    Returns:
+        bool: True while the book's judgment is pending or running.
+    """
+    return source_id in get_judge_task_state(app).running_book_ids
+
+
+def pop_book_judgment_outcome(app: FastAPI, source_id: int) -> str | None:
+    """Return and clear the outcome message from one book's last finished judgment.
+
+    Args:
+        app (FastAPI): App supplying the judge task state.
+        source_id (int): Book to fetch the outcome for.
+
+    Returns:
+        str | None: The outcome message, or None when there is nothing new to report.
+    """
+    return get_judge_task_state(app).outcome_messages.pop(source_id, None)