feat: add Settings and Media contexts with theme settings schema

- Create settings table for site-wide key-value configuration
- Create images table for BLOB storage of logo/header images
- Add Setting schema with JSON/string/integer/boolean support
- Add ThemeSettings embedded schema with all theme options
- Add Settings context with get/put/update operations
- Add Media context for image uploads and retrieval
- Add Image schema with SVG detection and storage
- Add 9 curated theme presets (gallery, studio, boutique, etc.)
- Add comprehensive tests for Settings and Media contexts
- Add seeds with default Studio preset
- All tests passing (29 tests, 0 failures)

priv/repo/seeds.exs

@@ -9,3 +9,12 @@
 #
 # We recommend using the bang functions (`insert!`, `update!`
 # and so on) as they will fail if something goes wrong.
+
+alias SimpleshopTheme.Settings
+
+# Set default theme settings (Studio preset)
+IO.puts("Setting up default theme settings...")
+
+{:ok, _theme} = Settings.apply_preset(:studio)
+
+IO.puts("✓ Default theme settings applied (Studio preset)")