Add YAML-based store configs with bidirectional sync

Stores can now be defined as YAML files in the stores/ directory.
On startup, YAML files are synced into the database. Changes made
via the admin UI are written back to YAML files automatically.

- Add store-sync service (load from files, export to files, write-back)
- Add /api/stores/sync and /api/stores/export endpoints
- Add Sync/Export buttons to admin UI
- Mount stores/ volume in Docker
- Include example store config template

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

stores/_example-store.yaml

@@ -0,0 +1,34 @@
+# Example store configuration for Price Hunter
+# Copy this file, rename it (the filename becomes the store slug), and fill in your values.
+#
+# Required fields: name, base_url, search_url, selectors (container, name, price, link)
+# The search_url must contain {query} where the search term will be inserted.
+#
+# Tips for finding CSS selectors:
+#   1. Open the store's search results page in your browser
+#   2. Right-click a product and choose "Inspect"
+#   3. Identify the repeating container element for each product card
+#   4. Find child elements for name, price, link, and image within that container
+#   5. Use the Test page in the admin UI to verify your selectors work
+
+name: Example Store
+base_url: https://www.example-store.com
+search_url: https://www.example-store.com/search?q={query}
+category: Electronics
+currency: EUR
+enabled: false
+
+selectors:
+  container: ".product-card"
+  name: ".product-title"
+  price: ".product-price"
+  link: "a.product-link"
+  image: "img.product-image"
+
+# Optional settings:
+# rate_limit: 2          # Max requests per second (default: 2)
+# rate_window: 1000      # Rate limit window in ms (default: 1000)
+# user_agent: "Custom UA" # Override the default browser user agent
+# proxy_url: "http://user:pass@proxy:8080"  # Route requests through a proxy
+# headers:               # Extra HTTP headers
+#   X-Custom-Header: "value"