feat(backend): fix T4 index drift, lazy import, FK policy, expand test coverage

backend/src/cyclone/db.py

@@ -11,6 +11,7 @@ same engine.
 from __future__ import annotations
 
 import enum
+import json
 import os
 from datetime import date, datetime
 from decimal import Decimal
@@ -28,6 +29,7 @@ from sqlalchemy import (
     String,
     Text,
     UniqueConstraint,
+    text,
 )
 from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column, relationship, sessionmaker
 from sqlalchemy.types import TypeDecorator
@@ -141,15 +143,11 @@ class JSONText(TypeDecorator):
     def process_bind_param(self, value, dialect):  # type: ignore[no-untyped-def]
         if value is None:
             return None
-        import json
-
         return json.dumps(value)
 
     def process_result_value(self, value, dialect):  # type: ignore[no-untyped-def]
         if value is None:
             return None
-        import json
-
         return json.loads(value)
 
 
@@ -172,8 +170,13 @@ class Batch(Base):
     )
 
     __table_args__ = (
-        Index("ix_batches_parsed_at", "parsed_at"),
+        # Migration is the source of truth: 0001_initial.sql declares this DESC.
+        # `text(...)` keeps the ORM in sync so fresh DBs built via `create_all`
+        # alone (no migration runner) get the same index direction as prod.
+        Index("ix_batches_parsed_at", text("parsed_at DESC")),
     )
+    # TODO(T6): when ActivityEvent model is added, mirror this pattern for
+    # `ix_activity_events_ts` on activity_events(ts DESC) per 0001_initial.sql:86.
 
 
 class Claim(Base):
@@ -196,7 +199,17 @@ class Claim(Base):
         Enum(ClaimState, native_enum=False), nullable=True
     )
     matched_remittance_id: Mapped[Optional[str]] = mapped_column(
-        String(64), ForeignKey("remittances.id"), nullable=True
+        # ORM policy: SET NULL on remittance delete. The claim may outlive its
+        # remittance during reversal/reimport flows; without this, deleting a
+        # remittance with matched claims raises IntegrityError.
+        # Note: 0001_initial.sql predates this policy (no ON DELETE clause).
+        # SQLite ignores FK direction by default unless PRAGMA foreign_keys=ON,
+        # so the inconsistency is benign there; the ORM is the source of truth
+        # for PostgreSQL/test environments with FK enforcement. Amendment to the
+        # migration is deferred to post-SQLite rollout.
+        String(64),
+        ForeignKey("remittances.id", ondelete="SET NULL"),
+        nullable=True,
     )
     raw_json: Mapped[Optional[dict]] = mapped_column(JSONText, nullable=True)