refactor: move backend files to backend/ directory

Reorganize project structure:
- backend/cmd/openteam/ — entry point
- backend/internal/ — core packages
- backend/middleware/ — HTTP middleware
- backend/router/ — route setup
- backend/wire/ — dependency injection
- backend/pkg/ — shared utilities
- backend/go.mod, go.sum — Go module files

Updated Makefile to work from backend/ directory.
Removed old lowercase makefile.

backend/go.mod

@@ -0,0 +1,71 @@
+module opencatd-open
+
+go 1.23.2
+
+require (
+	github.com/gin-contrib/cors v1.7.2
+	github.com/gin-gonic/gin v1.10.0
+	github.com/go-ozzo/ozzo-validation/v4 v4.4.1
+	github.com/go-webauthn/webauthn v0.12.3
+	github.com/golang-jwt/jwt/v5 v5.2.2
+	github.com/google/uuid v1.6.0
+	github.com/google/wire v0.6.0
+	github.com/joho/godotenv v1.5.1
+	github.com/lib/pq v1.10.9
+	github.com/pkoukk/tiktoken-go v0.1.7
+	github.com/sashabaranov/go-openai v1.42.0
+	github.com/spf13/cobra v1.9.1
+	golang.org/x/crypto v0.37.0
+	golang.org/x/time v0.10.0
+	gorm.io/driver/mysql v1.5.7
+	gorm.io/driver/postgres v1.5.11
+	gorm.io/driver/sqlite v1.6.0
+	gorm.io/gorm v1.30.0
+)
+
+require (
+	filippo.io/edwards25519 v1.1.0 // indirect
+	github.com/bytedance/sonic v1.13.2 // indirect
+	github.com/bytedance/sonic/loader v0.2.4 // indirect
+	github.com/cloudwego/base64x v0.1.5 // indirect
+	github.com/dlclark/regexp2 v1.11.4 // indirect
+	github.com/fxamacker/cbor/v2 v2.8.0 // indirect
+	github.com/gabriel-vasile/mimetype v1.4.8 // indirect
+	github.com/gin-contrib/sse v1.0.0 // indirect
+	github.com/go-playground/locales v0.14.1 // indirect
+	github.com/go-playground/universal-translator v0.18.1 // indirect
+	github.com/go-playground/validator/v10 v10.26.0 // indirect
+	github.com/go-sql-driver/mysql v1.8.1 // indirect
+	github.com/go-webauthn/x v0.1.20 // indirect
+	github.com/goccy/go-json v0.10.5 // indirect
+	github.com/google/go-cmp v0.7.0 // indirect
+	github.com/google/go-tpm v0.9.3 // indirect
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/jackc/pgpassfile v1.0.0 // indirect
+	github.com/jackc/pgservicefile v0.0.0-20221227161230-091c0ba34f0a // indirect
+	github.com/jackc/pgx/v5 v5.5.5 // indirect
+	github.com/jackc/puddle/v2 v2.2.1 // indirect
+	github.com/jinzhu/inflection v1.0.0 // indirect
+	github.com/jinzhu/now v1.1.5 // indirect
+	github.com/json-iterator/go v1.1.12 // indirect
+	github.com/klauspost/cpuid/v2 v2.2.10 // indirect
+	github.com/kr/text v0.2.0 // indirect
+	github.com/leodido/go-urn v1.4.0 // indirect
+	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/mattn/go-sqlite3 v1.14.22 // indirect
+	github.com/mitchellh/mapstructure v1.5.0 // indirect
+	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
+	github.com/modern-go/reflect2 v1.0.2 // indirect
+	github.com/pelletier/go-toml/v2 v2.2.3 // indirect
+	github.com/spf13/pflag v1.0.6 // indirect
+	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
+	github.com/ugorji/go/codec v1.2.12 // indirect
+	github.com/x448/float16 v0.8.4 // indirect
+	golang.org/x/arch v0.16.0 // indirect
+	golang.org/x/net v0.39.0 // indirect
+	golang.org/x/sync v0.13.0 // indirect
+	golang.org/x/sys v0.32.0 // indirect
+	golang.org/x/text v0.24.0 // indirect
+	google.golang.org/protobuf v1.36.6 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)