refactor: replace SQLite/GORM with JSON file storage

- Remove GORM and SQLite, use simple JSON file (data/certs.json) for persistence
- CertStore with sync.RWMutex for thread-safe in-memory cache
- All handlers updated to use config.Store instead of config.DB
- Cron job and stats updated accordingly
- go mod tidy removes unused gorm/sqlite dependencies

backend/go.mod

@@ -7,8 +7,6 @@ require (
 	github.com/gin-gonic/gin v1.9.1
 	github.com/go-acme/lego/v4 v4.14.2
 	github.com/robfig/cron/v3 v3.0.1
-	gorm.io/driver/sqlite v1.5.4
-	gorm.io/gorm v1.25.5
 )
 
 require (
@@ -28,14 +26,11 @@ require (
 	github.com/google/go-querystring v1.1.0 // indirect
 	github.com/hashicorp/go-cleanhttp v0.5.2 // indirect
 	github.com/hashicorp/go-retryablehttp v0.7.4 // indirect
-	github.com/jinzhu/inflection v1.0.0 // indirect
-	github.com/jinzhu/now v1.1.5 // indirect
 	github.com/jmespath/go-jmespath v0.4.0 // indirect
 	github.com/json-iterator/go v1.1.12 // indirect
 	github.com/klauspost/cpuid/v2 v2.2.5 // indirect
 	github.com/leodido/go-urn v1.2.4 // indirect
 	github.com/mattn/go-isatty v0.0.19 // indirect
-	github.com/mattn/go-sqlite3 v1.14.17 // indirect
 	github.com/miekg/dns v1.1.55 // indirect
 	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
 	github.com/modern-go/reflect2 v1.0.2 // indirect