feat(backend): add foundation services and sql idempotency

backend/command-api/sql/001_command_idempotency.sql

@@ -0,0 +1,13 @@
+CREATE TABLE IF NOT EXISTS command_idempotency (
+  composite_key TEXT PRIMARY KEY,
+  user_id TEXT NOT NULL,
+  route TEXT NOT NULL,
+  idempotency_key TEXT NOT NULL,
+  status_code INTEGER NOT NULL,
+  response_payload JSONB NOT NULL,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  expires_at TIMESTAMPTZ NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_command_idempotency_expires_at
+  ON command_idempotency (expires_at);