fix(kobo): version cover ids from the cover, not the metadata row

Any write to book_metadata bumped updated_at and minted a fresh
CoverImageId, so a device holding that cover locally fell back to the
placeholder offline. Stamp cover_updated_at at each cover write instead;
migration 0080 backfills it from updated_at so no id churns on upgrade.

Also trims the DECIMAL(10,2) scale Booklore hands back for a series
index, which sorted 2.50 after 2.9. Closes #943.

scripts/e2e/suite-registry.mjs

@@ -707,6 +707,25 @@ export const E2E_SUITES = Object.freeze({
       ...SHARED_DB_AND_HELPER_PATHS,
     ],
   },
+  "kobo-cover-identity": {
+    id: "kobo-cover-identity",
+    name: "Kobo Cover Identity",
+    timeout: 60,
+    lane: "full",
+    description: "Kobo CoverImageId stability across metadata edits and cover changes",
+    vitestTarget: "test/kobo-cover-identity.e2e-spec.ts",
+    junitOutput: `${TEST_RESULTS_DIR}/kobo-cover-identity-e2e-junit.xml`,
+    prepareDedicatedDatabase: true,
+    useDedicatedDatabase: true,
+    changedPaths: [
+      "server/src/modules/kobo/**",
+      "server/src/modules/cover/**",
+      "server/src/modules/metadata/**",
+      "server/test/kobo-cover-identity.e2e-spec.ts",
+      "server/test/e2e/reader-state-isolation/**",
+      ...SHARED_DB_AND_HELPER_PATHS,
+    ],
+  },
   "users-admin-lifecycle": {
     id: "users-admin-lifecycle",
     name: "Users Admin Lifecycle",