feat: extract MediaController, wire into Dispatcher, delete media.php

app/storage/migrations/007_system_cache.sql

@@ -0,0 +1,8 @@
+-- Migration 007: Add system_cache table for admin system page caching
+-- Stores JSON-encoded status snapshots keyed by section with a TTL mechanism.
+
+CREATE TABLE IF NOT EXISTS system_cache (
+    key        TEXT    PRIMARY KEY,
+    value      TEXT    NOT NULL,
+    updated_at INTEGER NOT NULL
+);