Phase 3: extract EvidenceService and rewrite the service ownership rule

Decompose SourceService along the aggregate boundary and then correct the
instruction file that caused it to grow, in that order. The refactor is the
empirical test of the rule.

services/evidence.py (new)
  EvidenceService owns ExecutionAttempt: read_latest_execution_attempt,
  list_execution_attempts, promote_machine_attempt, build_evidence_export,
  plus the LatestExecutionAttempt projection. Moved verbatim from sources.py.

services/errors.py (new)
  The five-class error hierarchy (PromptLoadError, TranscriptionError,
  TranscriptionNotFoundError, SourceDeleteBlockedError,
  CandidatePromotionError) moved out of sources.py. evidence.py needs
  TranscriptionNotFoundError, and test_service_boundaries.py correctly
  rejected the sibling import. errors.py defines no *Service class, so it is
  a legal shared home. This was the boundary test doing its job, not an
  obstacle to route around.

sources.py 1,389 -> 885 lines (1,063 after Phase 2).

services/__init__.py
  ServiceBundle and from_session_factory register evidence. Note that
  field-by-field ServiceBundle construction silently binds services to the
  process-global session factory via default_factory; from_session_factory is
  the only safe constructor. Two test bundles were fixed for this.

.github/instructions/services.instructions.md
  Rewritten to describe the boundaries the decomposition actually produced,
  per plan Phase 3 task 7 and review log [59].

  - "1 service class per data model" -> one service class per aggregate.
    The table-shaped rule is the measured cause of sources.py reaching
    1,389 lines; DocumentType has no lifecycle without Document.
  - New Model Ownership section. Junctions are owned by their lifecycle
    owner, the service that creates and deletes the rows: document_person
    to PeopleService (sole writer, measured), job_source to SourceService.
    Two carve-outs are stated rather than left as silent violations:
    cascade deletion when a service deletes its own aggregate root, and
    status transitions that create and delete nothing (cancel_job,
    resubmit_failed_sources), which are Job lifecycle events on the work
    queue. EvidenceService.promote_machine_attempt's two-field write to
    Source is named and scoped.
  - Mandatory CRUD softened to intent. It was already false: five modules
    define no service class, EvidenceService has no create/delete because
    ExecutionAttempt is append-only, RegistryService uses <op>_entry.
  - Separated reading across models via eager loads from the owning root,
    which is allowed, from importing another service, which is not. The old
    line 13 and lines 75-77 read as contradictory.
  - Typo: picutre.

  No code was moved to satisfy the rule.

tests/test_service_boundaries.py
  Docstring no longer cites the instruction file by line number; that anchor
  would desynchronise silently. errors.py added to the neutral-module list.

Verified: 292 passed, 4 skipped, 0 ruff, 0 ty. All 25 /ui/* routes walked
against the live app; 24x 200. /ui/documents/{id}/sources 404s via a 307 that
drops the /ui prefix, confirmed pre-existing (last touched in 6a3ee26) and
left alone as out of scope.

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

tests/test_service_boundaries.py

@@ -1,9 +1,10 @@
 """Structural rules for the services package.
 
-`.github/instructions/services.instructions.md:13` requires that service classes
-stay independent of one another. Shared behavior belongs in a neutral module
-(`base.py`, `registry.py`, `source_media.py`, `media_storage.py`), and any
-operation spanning two services belongs in an orchestration module.
+The "Structure" section of `.github/instructions/services.instructions.md` requires
+that service modules stay independent of one another. Shared behavior belongs in a
+neutral module that defines no service class (`base.py`, `errors.py`, `registry.py`,
+`source_media.py`, `media_storage.py`), and any operation that writes models owned by
+two services belongs in an orchestration module.
 """
 
 from __future__ import annotations
@@ -13,7 +14,7 @@ from pathlib import Path
 
 SERVICES_DIR = Path(__file__).resolve().parents[1] / "src" / "transcription" / "services"
 
-# Modules that intentionally compose several services rather than owning one table.
+# Modules that intentionally compose several services rather than owning one aggregate.
 ORCHESTRATION_MODULES = frozenset({"store", "workflows", "__init__"})