implement content-hash based image cache invalidation

Add content-based hashing to recipe images for proper cache invalidation
while maintaining graceful degradation through dual file storage.

Changes:
- Add imageHash utility with SHA-256 content hashing (8-char)
- Update Recipe model to store hashed filenames in images[0].mediapath
- Modify image upload endpoint to save both hashed and unhashed versions
- Update frontend components to use images[0].mediapath with fallback
- Add migration endpoint to hash existing images (production-only)
- Update image delete/rename endpoints to handle both file versions

Images are now stored as:
  - recipe.a1b2c3d4.webp (hashed, cached forever)
  - recipe.webp (unhashed, graceful degradation fallback)

Database stores hashed filename for cache busting, while unhashed
version remains on disk for backward compatibility and manual uploads.

src/utils/imageHash.ts

@@ -0,0 +1,50 @@
+import crypto from 'crypto';
+import fs from 'fs';
+
+/**
+ * Generates an 8-character hash from image file content
+ * Uses SHA-256 for reliable, content-based hashing
+ * @param filePath - Path to the image file
+ * @returns 8-character hex hash
+ */
+export function generateImageHash(filePath: string): string {
+  const fileBuffer = fs.readFileSync(filePath);
+  const hashSum = crypto.createHash('sha256');
+  hashSum.update(fileBuffer);
+  const hash = hashSum.digest('hex');
+  return hash.substring(0, 8);
+}
+
+/**
+ * Generates an 8-character hash from Buffer content
+ * @param buffer - Image file buffer
+ * @returns 8-character hex hash
+ */
+export function generateImageHashFromBuffer(buffer: Buffer): string {
+  const hashSum = crypto.createHash('sha256');
+  hashSum.update(buffer);
+  const hash = hashSum.digest('hex');
+  return hash.substring(0, 8);
+}
+
+/**
+ * Creates a filename with hash for cache busting
+ * @param basename - Base name without extension (e.g., "maccaroni")
+ * @param hash - 8-character hash
+ * @returns Filename with hash (e.g., "maccaroni.a1b2c3d4.webp")
+ */
+export function getHashedFilename(basename: string, hash: string): string {
+  return `${basename}.${hash}.webp`;
+}
+
+/**
+ * Extracts basename from a potentially hashed filename
+ * @param filename - Filename (e.g., "maccaroni.a1b2c3d4.webp" or "maccaroni.webp")
+ * @returns Basename without hash or extension (e.g., "maccaroni")
+ */
+export function extractBasename(filename: string): string {
+  // Remove .webp extension
+  const withoutExt = filename.replace(/\.webp$/, '');
+  // Remove hash if present (8 hex chars preceded by a dot)
+  return withoutExt.replace(/\.[a-f0-9]{8}$/, '');
+}