fix(index-builder): promote recent replays from B2 to R2 warm cache

Modified bundleWarmAssetsForCycle to return the deduplicated match IDs
of warm-set replays, and added a call to promoteRecentReplays in the
build cycle. This implements plan §8.2.5: index builder promotes recent
replays from B2 cold archive to R2 warm cache.

Acceptance criteria for plan-gap bead bf-jfmo:
- /r2/replays/{id}.json.gz will return valid gzipped replay JSON once
  the Cloudflare Pages R2 bucket binding (ACB_BUCKET) is configured
- R2 bucket exists with replay objects after each build cycle
- Index builder has R2 S3 client configured (credentials via env vars)

Closes: bf-jfmo

cmd/acb-index-builder/main.go

@@ -241,11 +241,20 @@ func runBuildCycle(ctx context.Context, db *sql.DB, cfg *Config) error {
 	}
 
 	// Bundle warm-set assets (replays, thumbnails, cards, evolution) from B2 into Pages deploy
-	if err := bundleWarmAssetsForCycle(ctx, db, cfg, data); err != nil {
+	warmMatchIDs, err := bundleWarmAssetsForCycle(ctx, db, cfg, data)
+	if err != nil {
 		slog.Error("Failed to bundle warm assets", "error", err)
 		// Non-fatal
 	}
 
+	// Promote recent replays from B2 to R2 warm cache (plan §8.2.5)
+	if len(warmMatchIDs) > 0 {
+		if err := promoteRecentReplays(ctx, cfg, warmMatchIDs); err != nil {
+			slog.Error("Failed to promote replays to R2", "error", err)
+			// Non-fatal
+		}
+	}
+
 	// Enrich featured replays with AI commentary (§13.3)
 	if err := enrichReplays(ctx, data, cfg, llmClient); err != nil {
 		slog.Error("Failed to enrich replays", "error", err)
@@ -263,16 +272,17 @@ func runBuildCycle(ctx context.Context, db *sql.DB, cfg *Config) error {
 
 // bundleWarmAssetsForCycle bundles warm-set assets from B2 into the Pages deploy directory.
 // This includes replays, thumbnails, bot cards, and evolution live.json.
-func bundleWarmAssetsForCycle(ctx context.Context, db *sql.DB, cfg *Config, data *IndexData) error {
+// Returns the deduplicated match IDs that were bundled (for R2 promotion).
+func bundleWarmAssetsForCycle(ctx context.Context, db *sql.DB, cfg *Config, data *IndexData) ([]string, error) {
 	// Get recent match IDs from the last 24 hours
 	recentMatchIDs, err := fetchRecentMatchIDs(ctx, db, 24*time.Hour)
 	if err != nil {
-		return fmt.Errorf("fetch recent match IDs: %w", err)
+		return nil, fmt.Errorf("fetch recent match IDs: %w", err)
 	}
 
 	if len(recentMatchIDs) == 0 {
 		slog.Debug("No recent matches to bundle")
-		return nil
+		return nil, nil
 	}
 
 	// Get exempt match IDs (playlists, series, seasons) - these are also part of warm set
@@ -300,7 +310,7 @@ func bundleWarmAssetsForCycle(ctx context.Context, db *sql.DB, cfg *Config, data
 
 	if len(dedupedMatchIDs) == 0 {
 		slog.Debug("No matches to bundle")
-		return nil
+		return nil, nil
 	}
 
 	slog.Info("Bundling warm assets from B2",
@@ -311,7 +321,7 @@ func bundleWarmAssetsForCycle(ctx context.Context, db *sql.DB, cfg *Config, data
 	// Create B2 client for bundling
 	b2Client, err := getB2Client(cfg)
 	if err != nil {
-		return fmt.Errorf("create B2 client: %w", err)
+		return nil, fmt.Errorf("create B2 client: %w", err)
 	}
 
 	// Bundle replays
@@ -344,7 +354,7 @@ func bundleWarmAssetsForCycle(ctx context.Context, db *sql.DB, cfg *Config, data
 		// Continue
 	}
 
-	return nil
+	return dedupedMatchIDs, nil
 }
 
 // fetchRecentMatchIDs retrieves match IDs from the last duration