Add file fingerprinting for move-resilient media item identity

Computes a SHA-256 partial-content fingerprint (file size + first 64 KB) for
movies, TV episodes, and mixed files during scans. When a file is moved or
renamed within a library, the scan detects the fingerprint match, renames the
media_items row in-place, and updates media_tags.media_key to match — so tags
and NFO metadata survive the move transparently.

- src/lib/fingerprint.ts: new computeFingerprint() using sync FS reads
- src/lib/db.ts: fingerprint TEXT column + index migration
- src/lib/tags.ts: reKeyMediaItem() to update media_tags on rename
- src/lib/scanner.ts: replace clear+upsert with detectMoves/reconcileAndPrune
  for movies, TV episodes, and mixed files; games retain clear+upsert (v1)
- TV scan restructured to a single filesystem pass (no double-scanning)

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

src/lib/fingerprint.ts

@@ -0,0 +1,36 @@
+import fs from 'fs'
+import crypto from 'crypto'
+
+const CHUNK_SIZE = 64 * 1024 // 64 KB
+
+/**
+ * Computes a stable partial-content fingerprint for a file.
+ * Uses SHA-256 of the file size + first 64 KB of content.
+ * Fast enough for large video files (~instant) and collision-resistant
+ * for real-world media libraries.
+ *
+ * Returns null if the file cannot be read (missing, permission error, etc.).
+ */
+export function computeFingerprint(absolutePath: string): string | null {
+  try {
+    const stat = fs.statSync(absolutePath)
+    const size = stat.size
+    const chunkLen = Math.min(CHUNK_SIZE, size)
+    const buf = Buffer.alloc(chunkLen)
+    if (chunkLen > 0) {
+      const fd = fs.openSync(absolutePath, 'r')
+      try {
+        fs.readSync(fd, buf, 0, chunkLen, 0)
+      } finally {
+        fs.closeSync(fd)
+      }
+    }
+    return crypto
+      .createHash('sha256')
+      .update(`${size}:`)
+      .update(buf)
+      .digest('hex')
+  } catch {
+    return null
+  }
+}