feat: implement unified S3-compatible storage system

Consolidates storage backends into a single S3-compatible driver that supports:
- AWS S3 (native)
- Backblaze B2 (S3-compatible API)
- Cloudflare R2 (S3-compatible API)
- MinIO and other S3-compatible services
- Local filesystem for development

This replaces the previous separate B2 driver with a unified approach,
reducing dependencies and complexity while adding support for more services.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

go.mod

@@ -5,6 +5,7 @@ go 1.24.2
 require (
 	github.com/Masterminds/semver v0.0.0-20180807142431-c84ddcca87bf
 	github.com/ant0ine/go-json-rest v0.0.0-20170913041208-ebb33769ae01
+	github.com/aws/aws-sdk-go v1.44.0
 	github.com/go-sql-driver/mysql v1.8.1
 	github.com/gorilla/websocket v0.0.0-20180605202552-5ed622c449da
 	github.com/mailgun/mailgun-go/v4 v4.3.0
@@ -18,6 +19,7 @@ require (
 require (
 	github.com/fsnotify/fsnotify v1.8.0 // indirect
 	github.com/go-viper/mapstructure/v2 v2.2.1 // indirect
+	github.com/jmespath/go-jmespath v0.4.0 // indirect
 	github.com/mattn/go-sqlite3 v1.14.22 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.3 // indirect
 	github.com/sagikazarmark/locafero v0.7.0 // indirect
@@ -42,7 +44,7 @@ require (
 	github.com/json-iterator/go v1.1.10 // indirect
 	github.com/modern-go/concurrent v0.0.0-20180228061459-e0a39a4cb421 // indirect
 	github.com/modern-go/reflect2 v0.0.0-20180701023420-4b7aa43c6742 // indirect
-	github.com/pkg/errors v0.8.1 // indirect
+	github.com/pkg/errors v0.9.1 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/stretchr/objx v0.5.2 // indirect
 	golang.org/x/text v0.21.0 // indirect