fix(series): repair legacy series index upgrades

Normalize legacy float values before migration 0076 so 2.7 databases can upgrade without constraint failures or precision loss. Serialize the compatibility conversion across concurrent startup processes.

Fixes #1189

scripts/e2e/suite-registry.mjs

@@ -304,6 +304,24 @@ export const E2E_SUITES = Object.freeze({
       ...SHARED_DB_AND_HELPER_PATHS,
     ],
   },
+  "series-index-migration": {
+    id: "series-index-migration",
+    name: "Series Index Migration",
+    timeout: 60,
+    lane: "full",
+    description: "Legacy series index upgrade compatibility suite",
+    vitestTarget: "test/series-index-migration.e2e-spec.ts",
+    junitOutput: `${TEST_RESULTS_DIR}/series-index-migration-e2e-junit.xml`,
+    prepareDedicatedDatabase: true,
+    useDedicatedDatabase: true,
+    changedPaths: [
+      "server/src/db/migrations/**",
+      "server/src/scripts/migrate.ts",
+      "server/src/scripts/series-index-migration-compatibility.ts",
+      "server/test/series-index-migration.e2e-spec.ts",
+      ...SHARED_DB_AND_HELPER_PATHS,
+    ],
+  },
   "reading-attempts": {
     id: "reading-attempts",
     name: "Reading Attempts",