fix(cdk): upgrade RDS Postgres 14.5 -> 15.8 to match schema (NULLS NOT DISTINCT)

Migration 0029 adds `UNIQUE NULLS NOT DISTINCT` on point_event (the dedupe /
anti-gaming key), which is Postgres 15+ syntax. The RDS instance was pinned to
14.5, so `db:migrate` failed on every Vercel deploy. Bump the engine to 15.8
(local dev already runs postgres:15-alpine) and set allowMajorVersionUpgrade so
RDS accepts the major-version change.

NOTE: requires `cdk deploy` of the StorageStack to take effect (in-place RDS
major upgrade, brief downtime). Vercel stays red until that deploy completes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: NiallJoeMaher

cdk/lib/storage-stack.ts

@@ -145,7 +145,10 @@ export class StorageStack extends cdk.Stack {
       instanceIdentifier: "codu-rds",
       databaseName: dbName,
       engine: rds.DatabaseInstanceEngine.postgres({
-        version: rds.PostgresEngineVersion.VER_14_5,
+        // PG15+ is required by the schema (point_event dedupe uses
+        // `UNIQUE NULLS NOT DISTINCT`, added in Postgres 15). Local dev runs
+        // postgres:15-alpine; this keeps RDS in step.
+        version: rds.PostgresEngineVersion.VER_15_8,
       }),
       credentials: rds.Credentials.fromPassword(
         dbUsername,
@@ -164,6 +167,10 @@ export class StorageStack extends cdk.Stack {
       publiclyAccessible: true,
       deletionProtection: props?.production ?? false,
       autoMinorVersionUpgrade: true,
+      // Required for the 14.5 -> 15.x major-version upgrade; RDS rejects a
+      // major engine change without it. Safe to leave enabled (it permits, it
+      // doesn't force, future major upgrades via CDK).
+      allowMajorVersionUpgrade: true,
       backupRetention: props?.production
         ? cdk.Duration.days(7) // 7 days retention for production
         : cdk.Duration.days(1), // 1 day retention for non-production (minimum allowed)