Reduce code duplication and update README

- Extract shared utilities (HIDDEN_FILES, VIDEO_EXTENSIONS, fileApiUrl,
  thumbnailApiUrl, findFile) into new src/lib/media-utils.ts, removing
  identical copies from games.ts, movies.ts, tv.ts, files.ts, and scanner.ts
- Add comment in files.ts clarifying why its VIDEO_EXTENSIONS set intentionally
  differs from the media library set (web-playable formats for the mixed browser)
- Rewrite README to reflect the current feature set: Movies/TV libraries, auth
  system, tag system, background scanner, updated project structure, folder
  conventions for all four library types, and a complete API reference

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/media-utils.ts

@@ -0,0 +1,31 @@
+import fs from 'fs'
+import path from 'path'
+
+export const HIDDEN_FILES = /^\./
+
+/** Video extensions for dedicated media libraries (movies, TV). */
+export const VIDEO_EXTENSIONS = new Set(['.mkv', '.mp4', '.avi', '.mov', '.m4v', '.wmv', '.ts', '.m2ts'])
+
+export function fileApiUrl(libraryId: string, relativePath: string): string {
+  return `/api/file?libraryId=${encodeURIComponent(libraryId)}&path=${encodeURIComponent(relativePath)}`
+}
+
+export function thumbnailApiUrl(libraryId: string, relativePath: string): string {
+  return `/api/thumbnail?libraryId=${encodeURIComponent(libraryId)}&path=${encodeURIComponent(relativePath)}`
+}
+
+/**
+ * Finds the first non-hidden file in a directory whose basename (without extension)
+ * matches the given pattern (case-insensitive).
+ */
+export function findFile(dir: string, pattern: RegExp): string | null {
+  let entries: string[]
+  try {
+    entries = fs.readdirSync(dir)
+  } catch {
+    return null
+  }
+  return entries.find(
+    (e) => !HIDDEN_FILES.test(e) && pattern.test(path.basename(e, path.extname(e)))
+  ) ?? null
+}