Fix inflated watch metrics: one row per viewing instead of per 30s ping

The player pings /track/watch every 30s with the CUMULATIVE position, and each
ping inserted a new row (~18 rows per viewing, 254k/day). SUM(seconds_watched)
therefore summed cumulative snapshots: a single 58-minute viewing reported as
4238 minutes (~73x), skewing TrendingController's watch-minute ranking and the
admin "total_sec"/"plays" figures.

Upsert per (session_id, episode_id) so SUM is the real watch time and COUNT is
the real play count. Adds the supporting index.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

database/migrations/2026_07_15_000001_add_session_episode_index_to_watch_events.php

@@ -0,0 +1,27 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration {
+    /**
+     * TrackingController::watch() artık izleme (session + bölüm) başına tek
+     * satır tutup güncelliyor; bu index olmadan her ping tabloyu tarardı.
+     */
+    public function up(): void
+    {
+        if (! Schema::hasIndex('analytics_watch_events', 'analytics_watch_events_session_id_episode_id_index')) {
+            Schema::table('analytics_watch_events', function (Blueprint $table) {
+                $table->index(['session_id', 'episode_id']);
+            });
+        }
+    }
+
+    public function down(): void
+    {
+        Schema::table('analytics_watch_events', function (Blueprint $table) {
+            $table->dropIndex('analytics_watch_events_session_id_episode_id_index');
+        });
+    }
+};