V4.6 Phase 6: async I/O and configuration hygiene

MED-01 - move remaining blocking work off the event loop:
- normalization.py gains normalize_orientation_async; the Pillow decode,
  transpose, and re-encode now run via asyncio.to_thread. The sync entry point
  stays for tests and documents that it blocks.
- OrientationNormalization.digest_sha256 becomes a stored field computed inside
  normalize_orientation, which already runs off-loop, instead of a property that
  hashed page-sized derivative bytes on the caller's thread.
- SourceService._write_and_digest_artifact performs the artifact write and its
  sha256 in a single worker-thread hop; both external-artifact write sites are
  now dispatched through to_thread.
- transcribe_image dispatches load_source_payload and build_prompt_execution
  through to_thread.

MED-04 - replace functools.cache on the engine and session factories with
explicit URL-keyed registries. dispose_engine and dispose_session_factory now
evict only the requested URL; previously cache_clear() tore down every other
database in the process, and dispose_engine would construct an engine for an
unknown URL purely to throw it away. New tests/test_engine_registry.py covers
distinct engines per URL, targeted eviction, and the unknown-URL no-op.

config.py - replace object.__setattr__ in normalize_provider_models with a
model_validator(mode="before") over the raw input, so the derived selector is
produced by normal construction rather than by mutating a frozen instance.
model_copy(update=...) was tried first and rejected: pydantic-settings does not
support a top-level validator returning anything other than self when validating
via __init__. provider_model is now stripped as well as the tuple entries.

models.py - add onupdate to the five updated_at columns and to Job.date_updated,
and drop the 10 manual "updated_at = datetime.now(UTC)" assignments across the
document, job, people, registry, and source services. Verified DDL-neutral by
hashing CreateTable output for every table on both the sqlite and postgresql
dialects before and after: identical, so this stays in Phase 6 and Phase 2 does
not need re-verification. New tests/services/test_timestamps.py asserts an
update through each service advances the timestamp.

MED-08 - Job.filename no longer swallows every exception to None. Relationships
declare lazy="raise", so the new _loaded_attribute helper inspects load state
explicitly and returns None only for genuinely unloaded attributes; real errors
now surface. Job.error_detail uses the same helper, which also removes its
unguarded read of the lazy="raise" job_sources relationship.

Verification: ruff check src tests clean; 288 passed, 4 skipped.

Co-authored-by: Copilot App <[email protected]>

tests/services/test_normalization.py

@@ -19,6 +19,7 @@ from transcription.services import ServiceBundle
 from transcription.services.documents import DocumentService
 from transcription.services.jobs import JobService
 from transcription.services.normalization import normalize_orientation
+from transcription.services.normalization import normalize_orientation_async
 from transcription.services.sources import SourceService
 from transcription.services.workflows import process_queued_job
 
@@ -226,3 +227,18 @@ async def test_worker_sends_exact_derivative_and_links_attempt_evidence(
         "transcription_quality_warnings",
     }
     assert {artifact.execution_attempt_id for artifact in artifacts} == {attempts[0].id}
+
+
+@pytest.mark.asyncio
+async def test_async_wrapper_matches_sync_result_and_precomputes_digest(tmp_path):
+    """[MED-01]: Pillow work runs off the event loop and hashes its own output."""
+    path = tmp_path / "async-upside-down.jpg"
+    _write_oriented_jpeg(path, orientation=3)
+
+    result = await normalize_orientation_async(path, media_type="image/jpeg")
+    expected = normalize_orientation(path, media_type="image/jpeg")
+
+    assert result is not None
+    assert expected is not None
+    assert result.content == expected.content
+    assert result.digest_sha256 == hashlib.sha256(result.content).hexdigest()