feat: wire Resend mail and Cloudflare R2 storage (#4)

Add production provider deps/config so transactional email and CMS
media can use Resend and a dedicated r2 disk with custom-domain URLs.

Co-authored-by: Cursor <cursoragent@cursor.com>

openspec/changes/add-production-providers/specs/content-media/spec.md

@@ -0,0 +1,20 @@
+## ADDED Requirements
+
+### Requirement: CMS public image uploads select production object storage disk
+
+Public content image uploads (Filament FileUpload via `PublicImageUploadRules`) MUST store files on the disk matching the configured default filesystem (SPEC §6.4). When `FILESYSTEM_DISK` is `r2`, uploads MUST use the `r2` disk. When `FILESYSTEM_DISK` is `s3`, uploads MUST use the `s3` disk. Otherwise uploads MUST use the local `public` disk.
+
+#### Scenario: Production R2 disk is used for CMS uploads
+
+- **WHEN** `FILESYSTEM_DISK=r2`
+- **THEN** `PublicImageUploadRules::disk()` MUST return `r2`
+
+#### Scenario: Legacy S3 default still supported
+
+- **WHEN** `FILESYSTEM_DISK=s3`
+- **THEN** `PublicImageUploadRules::disk()` MUST return `s3`
+
+#### Scenario: Local development uses public disk
+
+- **WHEN** `FILESYSTEM_DISK` is `local`, unset, or any value other than `r2`/`s3`
+- **THEN** `PublicImageUploadRules::disk()` MUST return `public`