kernel: real SqliteMetadataStore — SQLAlchemy models for all entities + initial migration

[JArmandoAnaya]

Context

The bootstrap left SqliteMetadataStore as engine + empty schema creation. All M1 services
need real persistence for the full entity set.

Deliverables

• SQLAlchemy table models for: Workspace metadata, Project, AnnotationSchema (+ LabelClass,
  Attribute as structured JSON or child tables — decide and document), Source, IngestJob,
  Asset, Batch, TaskGroup, AnnotationJob, Annotation, Dataset membership + change log,
  Release (+ Manifest reference).
• Mapping layer between SQLAlchemy rows and the pydantic domain models (the domain never
  imports SQLAlchemy types in its signatures).
• Initial schema creation as migration 001 (alembic or a minimal versioned-DDL mechanism —
  decide here; the workspace format_version must gate it).
• Transaction scope helper: a unit-of-work per Project-aggregate operation.

Acceptance criteria

• Round-trip tests: every entity persists and rehydrates equal (UUIDs, enums, JSON fields)
• Annotation stores UUID PK, provenance, schema_version, geometry JSON
• A fresh workspace creates schema at format_version=1; reopening validates it
• mypy strict stays green on the kernel

[JArmandoAnaya]

Done: Tables for all 14 entities + private mapping layer (_tables.py/_mappers.py), generic Repository[T] + UnitOfWork on the port, migration 001 gated by format_version, kernel/errors.py, DatasetMember/DatasetChange shapes, docs/persistence.md — PR #74.
Decisions: hand-rolled versioned DDL over alembic (FORMAT_VERSION derived from the MIGRATIONS list, so it cannot drift from a second ledger); immutable nested values as JSON (classes, geometry, manifest) vs. child tables for the two real relations (batch_asset with position, annotation_job_asset); timestamps as ISO-8601 TEXT with offset because SQLite's DATETIME drops the timezone; PRAGMA foreign_keys = ON per connection, with a test that fails without it.
Deferred: on-disk workspace layout and open() validation stay in #4 — #3 owns format_version inside the database file only. Uniqueness beyond the PK (project names, schema versions) is the services'.
Follow-ups: none.
Next: #4 — WorkspaceService: init/open on disk, format_version, project-name uniqueness.