feat: add local SQLite cache with background sync and HA deployment

- Add SyncEngine for background sync between local SQLite cache and
  remote API with pending_ops queue for offline resilience
- Refactor MCP server to support three modes: SQLite-only, hybrid
  (local cache + sync, new default), and HTTP-only (legacy)
- Add GET /api/memories/sync endpoint for incremental sync
- Change DELETE to soft delete (set deleted_at) for sync support
- Add deleted_at IS NULL filters to all read queries
- Scale API deployment to 2 replicas with pod anti-affinity, PDB,
  and startup probe for high availability
- Add migration 003 for deleted_at column and updated_at index
- Add comprehensive tests for sync engine and API sync endpoint

migrations/versions/003_add_soft_delete_and_sync.py

@@ -0,0 +1,40 @@
+"""Add soft delete and sync support.
+
+Revision ID: 003
+Revises: 002
+Create Date: 2026-03-14
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "003"
+down_revision: Union[str, None] = "002"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def _column_exists(conn, column_name: str) -> bool:
+    result = conn.execute(
+        sa.text(
+            "SELECT EXISTS(SELECT 1 FROM information_schema.columns "
+            "WHERE table_name = 'memories' AND column_name = :col)"
+        ),
+        {"col": column_name},
+    )
+    return result.scalar()
+
+
+def upgrade() -> None:
+    conn = op.get_bind()
+
+    if not _column_exists(conn, "deleted_at"):
+        op.add_column("memories", sa.Column("deleted_at", sa.DateTime(timezone=True), nullable=True))
+
+    op.execute("CREATE INDEX IF NOT EXISTS idx_memories_updated ON memories(updated_at)")
+
+
+def downgrade() -> None:
+    op.drop_index("idx_memories_updated")
+    op.drop_column("memories", "deleted_at")