Add core data model (12 tables) and initial Alembic migration

All core entities from ARCHITECTURE §5: tenancy (User, Tree, TreeMembership), people (Person, Name, Relationship), facts (Event, Place, PlaceName), provenance (Source, Citation), and the append-only AuditEntry. Cross-cutting mixins give every row a UUID key, timestamps, soft delete, and (where tree-owned) a tree_id for uniform tenant isolation.

Modeling choices: parentage as qualified edges (biological/adoptive/step/foster/donor/guardian) so non-traditional families are first-class; events keep both a verbatim date string and a normalized start/end range; closed sets are PG enums while GEDCOM-extensible vocabularies (event/name/source type) stay strings; CHECK constraints enforce single-subject events and single-target citations. Place is tree-scoped in Phase 0 (see ARCHITECTURE note). The migration is verified reversible (upgrade/downgrade drops tables and enum types) and matches the models (alembic check clean); applied on the deploy target. Dockerfile now ships migrations.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/models/mixins.py

@@ -0,0 +1,45 @@
+"""Reusable column mixins.
+
+- ``UUIDPrimaryKey`` — UUID surrogate key (no PII in URLs; safe for multi-tenant).
+- ``Timestamps``     — created/updated audit timestamps (DB-managed).
+- ``SoftDelete``     — ``deleted_at``; a row is "deleted" when set. A scheduled
+                       worker purges rows past the 30-day window (PRD US-080/081).
+- ``TenantScoped``   — ``tree_id`` FK; every tree-owned row carries it so the
+                       privacy engine can enforce isolation uniformly.
+"""
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, func
+from sqlalchemy.orm import Mapped, declared_attr, mapped_column
+
+
+class UUIDPrimaryKey:
+    id: Mapped[uuid.UUID] = mapped_column(primary_key=True, default=uuid.uuid4)
+
+
+class Timestamps:
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), nullable=False
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        onupdate=func.now(),
+        nullable=False,
+    )
+
+
+class SoftDelete:
+    deleted_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True, default=None
+    )
+
+
+class TenantScoped:
+    @declared_attr
+    def tree_id(cls) -> Mapped[uuid.UUID]:  # noqa: N805
+        return mapped_column(
+            ForeignKey("trees.id", ondelete="CASCADE"), nullable=False, index=True
+        )