Add support for S3-backed image storage

migrations/20261011120000_image_uploads.sql

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+CREATE TABLE image_uploads (
+    id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
+    -- Discord user ID from the authenticated session, including guild-only users.
+    uploader BIGINT UNSIGNED NOT NULL,
+    url VARCHAR(512) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL UNIQUE,
+    created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    orphaned_at DATETIME DEFAULT NULL,
+    INDEX image_uploads_uploader (uploader),
+    INDEX image_uploads_orphaned_at (orphaned_at)
+);