docs: codify the FK-safe stream ordering as a wire-format invariant

The postgres importer inserts records sequentially with FK enforcement live,
so it depends on records arriving in dependency order. That requirement was
implicit in each backend's export table list; make it explicit and enforced.

- Add EXPORT_KIND_ORDER (user, abode, resident, apikey, note) as a documented
  single source of truth, with the FK dependency chain spelled out on its doc
  comment, and note the ordering guarantee on the ExportEnvelope wire-format
  doc. Derive the isExportKind set from it.
- Both backends' export() now iterate EXPORT_KIND_ORDER via a loader map
  (postgres omits note by leaving it out of the map), so emission order is
  tied to the constant and can't drift.
- Tests: a change-detector on EXPORT_KIND_ORDER, plus assertions that both the
  sqlite and postgres (fake) exports emit record kinds grouped in FK-safe
  order (kind rank non-decreasing down the stream, after the leading meta).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/backends/postgres/export-import.test.ts

@@ -4,6 +4,10 @@ import { Readable } from "node:stream";
 import { PostgresInterface } from "../../../src/db/postgres/PostgresInterface.js";
 import type { WrappedPgClient } from "../../../src/db/postgres/pool.js";
 import type { SqlCode } from "../../../src/db/postgres/sql.js";
+import {
+  EXPORT_KIND_ORDER,
+  type ExportKind,
+} from "../../../src/db/types/ExportImport.js";
 
 // The postgres backend has no CI database, so these tests drive the real
 // PostgresInterface.export/import code paths against an in-memory fake client.
@@ -158,6 +162,20 @@ describe("postgres export", () => {
     assert.ok(!kinds.has("note"), "note is unsupported on postgres");
   });
 
+  it("emits record kinds grouped in FK-safe EXPORT_KIND_ORDER", async () => {
+    const db = new PostgresInterface(new FakePg(seededRows()));
+    const lines = parseLines(await streamToString(db.export()));
+    assert.equal(lines[0]?.kind, "meta", "first line is meta");
+    const rank = (k: string) => EXPORT_KIND_ORDER.indexOf(k as ExportKind);
+    let last = -1;
+    for (const { kind } of lines.slice(1)) {
+      const r = rank(kind);
+      assert.notEqual(r, -1, `unexpected kind ${kind}`);
+      assert.ok(r >= last, `kind ${kind} out of FK-safe order`);
+      last = r;
+    }
+  });
+
   it("applies the kinds filter", async () => {
     const db = new PostgresInterface(new FakePg(seededRows()));
     const lines = parseLines(