feat(backup): admin-tunable retention + BACKUP_KEY as a Komodo secret

Retention (keep-last / keep-daily-days) is operational policy the on-site admin
should tune, not a server env var requiring a redeploy -- same reasoning that moved
the target directory to the UI.

- Migration 0017: site_config.backup_keep_last + backup_keep_daily_days (nullable;
  null = code default 7 / 30 per field).
- BackupService reads retention fresh each run; status() exposes keepLast +
  keepDailyDays. DEFAULT_BACKUP_RETENTION is now a pure code default (env reads gone).
- PUT /api/backup/config accepts keepLast / keepDailyDays (non-negative int, or null
  to reset to default; 400 on negative).
- UI: two retention fields on the Backup config card; one Save covers target +
  retention. i18n sq + en.

BACKUP_KEY wired into Komodo:
- komodo/resources.toml: BACKUP_KEY=[[park_buzi_backup_key]] (per-booth secret,
  alongside JWT / signing keys).
- komodo/.env.komodo.example: documents it as the ONLY backup env var -- escrow it
  offsite alongside EVENT_SIGNING_KEY (recovery needs both); target + retention are
  admin-chosen in the UI / DB, not env. Server .env.example trimmed to just BACKUP_KEY.

Also carries the small in-progress setup-intro i18n copy trim.

Tests: 218 server tests green, incl. retention persist / reset-to-default / reject-
negative and the updated status shape. Migration applies cleanly (needed a
statement-breakpoint between the two ALTERs). Wiki backup-recovery updated.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

apps/server/src/routes/backup.routes.test.ts

@@ -50,6 +50,8 @@ describe("GET /api/backup/status", () => {
     expect(body).toMatchObject({
       configured: false,
       targetDir: null,
+      keepLast: 7, // code defaults surfaced when unset
+      keepDailyDays: 30,
       running: false,
       lastSuccessAt: null,
       lastError: null,
@@ -99,6 +101,37 @@ describe("PUT /api/backup/config — admin-chosen target", () => {
     });
     expect(clear.json().targetDir).toBeNull();
   });
+
+  it("persists retention and resets to defaults on null", async () => {
+    const { username, password } = await seedUser(db, { username: "boss", roleId: "admin" });
+    const { cookie, csrf } = await login(app, username, password);
+
+    const set = await app.inject({
+      method: "PUT", url: "/api/backup/config",
+      headers: { cookie, "x-csrf-token": csrf },
+      payload: { keepLast: 3, keepDailyDays: 14 },
+    });
+    expect(set.json()).toMatchObject({ keepLast: 3, keepDailyDays: 14 });
+
+    // null resets to the code default.
+    const reset = await app.inject({
+      method: "PUT", url: "/api/backup/config",
+      headers: { cookie, "x-csrf-token": csrf },
+      payload: { keepLast: null, keepDailyDays: null },
+    });
+    expect(reset.json()).toMatchObject({ keepLast: 7, keepDailyDays: 30 });
+  });
+
+  it("rejects a negative retention value (400)", async () => {
+    const { username, password } = await seedUser(db, { username: "boss", roleId: "admin" });
+    const { cookie, csrf } = await login(app, username, password);
+    const res = await app.inject({
+      method: "PUT", url: "/api/backup/config",
+      headers: { cookie, "x-csrf-token": csrf },
+      payload: { keepLast: -1 },
+    });
+    expect(res.statusCode).toBe(400);
+  });
 });
 
 describe("POST /api/backup/test — path probe", () => {