fix: prevent immutable cache headers on file access errors

When a file exists in the database as storedInternal but is missing from
disk, the error response was incorrectly getting Cache-Control:
max-age=31536000, immutable. This was because the handler set immutable
cache headers before attempting to stream the file, and by the time the
stream encountered the ENOENT error, HTTP headers were already committed.

Fix: add fs.promises.access() check in FileServerFileResolver for stored
original files before returning the path. If the file is not accessible
on disk, the access check throws, propagating to errorHandler which
correctly sets Cache-Control: max-age=300.

For thumbnail/webpublic stored files, detectType() already performed this
check implicitly via fs.stat(). Only original files were affected.

This fixes both the /files/:key and /proxy/:url* endpoints since both
call resolveFileByAccessKey for local files.

Agent-Logs-Url: https://github.com/misskey-dev/misskey/sessions/5dd1c781-b9ad-43cf-b107-79c502d2e602

Co-authored-by: syuilo <4439005+syuilo@users.noreply.github.com>

packages/backend/test/unit/server/FileServerService.ts

@@ -479,6 +479,24 @@ describe('FileServerService', () => {
 			expect(res.headers['cache-control']).toBe('max-age=86400');
 		});
 
+		test('GET /files/:key DBに存在するがストレージ上に存在しないファイルにアクセスしたときエラーのキャッシュをimmutableにしない', async () => {
+			const accessKey = randomString();
+			// Insert into DB as storedInternal but do NOT write the file to disk
+			await insertDriveFile({
+				accessKey,
+				storedInternal: true,
+				isLink: false,
+			});
+
+			const res = await fastify.inject({
+				method: 'GET',
+				url: `/files/${accessKey}`,
+			});
+
+			expect(res.statusCode).toBe(500);
+			expect(res.headers['cache-control']).toBe('max-age=300');
+		});
+
 		test('GET /files/:key 外部リンクを取得して配信する', async () => {
 			const accessKey = randomString();
 			await insertDriveFile({
@@ -766,5 +784,26 @@ describe('FileServerService', () => {
 			expect(res.headers['cache-control']).toBe('max-age=31536000, immutable');
 			expect(res.headers['content-disposition'] ?? '').toContain('dummy.png');
 		});
+
+		test('GET /proxy/:url* DBに存在するがストレージ上に存在しないファイルにアクセスしたときエラーのキャッシュをimmutableにしない', async () => {
+			const accessKey = randomString();
+			// Insert into DB as storedInternal but do NOT write the file to disk
+			await insertDriveFile({
+				accessKey,
+				storedInternal: true,
+				isLink: false,
+			});
+
+			const res = await fastify.inject({
+				method: 'GET',
+				url: `/proxy/any?url=${encodeURIComponent(`${config.url}/files/${accessKey}`)}&origin=1`,
+				headers: {
+					'user-agent': 'Mozilla/5.0',
+				},
+			});
+
+			expect(res.statusCode).toBe(500);
+			expect(res.headers['cache-control']).toBe('max-age=300');
+		});
 	});
 });