T01: core infrastructure + auth + multi-tenant + RLS

- 10 models: tenants, users, user_tenants, roles, sessions, audit_log, deletion_log, notifications, password_reset_tokens, api_tokens
- Session-based auth (Redis + PostgreSQL audit trail)
- Multi-tenant with ORM-level filtering + PostgreSQL RLS (set_config)
- RBAC with roles/permissions + field-level permissions
- CSRF protection via Origin header validation
- Auth rate limiting (Redis counters with TTL)
- CORS with explicit origins (no wildcard)
- Health endpoint (no auth required)
- Notification service + audit log middleware
- 29 tests, 26 ACs, all passing
- Coverage: 62% (infrastructure modules pending coverage in later tasks)

app/models/session.py

@@ -0,0 +1,30 @@
+"""Session model — PostgreSQL audit trail for sessions."""
+
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import String, DateTime, ForeignKey, func
+from sqlalchemy.dialects.postgresql import UUID as PGUUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.core.db import Base, TenantMixin
+
+
+class Session(Base, TenantMixin):
+    """Immutable session audit record. Runtime session lookup uses Redis."""
+
+    __tablename__ = "sessions"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True), ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    csrf_token: Mapped[str] = mapped_column(String(255), nullable=False)
+    expires_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )