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_timestamps.py

@@ -0,0 +1,68 @@
+"""Phase 6 verification that modification timestamps advance automatically.
+
+Covers the `onupdate` change: `updated_at` / `date_updated` are now maintained
+by the ORM column default rather than by hand at each call site, so update paths
+that previously forgot to set them no longer report a stale timestamp.
+"""
+
+from uuid import uuid4
+
+import pytest
+
+from transcription.db.models import Document
+from transcription.db.models import Job
+from transcription.db.models import JobStatus
+from transcription.db.models import Person
+from transcription.services.documents import DocumentService
+from transcription.services.jobs import JobService
+from transcription.services.people import PeopleService
+from transcription.services.people import PersonRoleRegistry
+
+
+@pytest.mark.asyncio
+async def test_document_update_advances_updated_at(default_session_factory):
+    documents = DocumentService(session_factory=default_session_factory)
+    document = await documents.create_document(Document(id=uuid4(), name="timestamps"))
+    original = document.updated_at
+
+    document.name = "timestamps renamed"
+    updated = await documents.update_document(document)
+
+    assert updated.updated_at > original
+
+
+@pytest.mark.asyncio
+async def test_person_update_advances_updated_at(default_session_factory):
+    people = PeopleService(session_factory=default_session_factory)
+    person = await people.create_person(Person(full_name="Grace Hopper"))
+    original = person.updated_at
+
+    person.full_name = "Rear Adm. Grace Hopper"
+    updated = await people.update_person(person)
+
+    assert updated.updated_at > original
+
+
+@pytest.mark.asyncio
+async def test_registry_update_advances_updated_at(default_session_factory):
+    roles = PersonRoleRegistry(session_factory=default_session_factory)
+    role = await roles.create_entry(label="Witness")
+    original = role.updated_at
+
+    updated = await roles.update_entry(role.id, label="Chief Witness", is_active=True)
+
+    assert updated.updated_at > original
+
+
+@pytest.mark.asyncio
+async def test_job_status_update_advances_date_updated(default_session_factory):
+    documents = DocumentService(session_factory=default_session_factory)
+    jobs = JobService(session_factory=default_session_factory)
+
+    document = await documents.create_document(Document(id=uuid4(), name="job-timestamps"))
+    job = await jobs.create_job(Job(document_id=document.id))
+    original = job.date_updated
+
+    updated = await jobs.update_job_state(job_id=job.id, status=JobStatus.PROCESSING)
+
+    assert updated.date_updated > original