refactor(backend): migrate to modular repository pattern with separated concerns

- Extract database layer into model and repo packages
- Split repository into focused modules (control, federation, flow, groups, mutations)
- Remove monolithic db.go and sqlite/repository.go
- Update handlers to use new repository structure
- Migrate contract tests to new patterns
- Add migration plan documentation

go-backend/AGENTS.md

@@ -2,7 +2,7 @@
 
 ## OVERVIEW
 Go-based Admin API for FLVX. Replaced legacy Spring Boot backend.
-**Stack:** Go 1.23, net/http (std lib), SQLite/PostgreSQL (modernc.org/sqlite - CGO-free).
+**Stack:** Go 1.23, net/http (std lib), GORM + SQLite/PostgreSQL (glebarez/sqlite - CGO-free).
 
 ## STRUCTURE
 ```
@@ -14,9 +14,14 @@ go-backend/
 │   │   ├── handler/          # API Handlers (User, Tunnel, Node, etc.)
 │   │   ├── middleware/       # JWT, CORS, Logging, Recover
 │   │   └── response/         # JSON response helpers
-│   ├── store/sqlite/         # Data Access Layer (Repository pattern)
-│   │   ├── repository.go     # SQL queries & Struct definitions
-│   │   └── sql/              # Embedded schema.sql & data.sql
+│   ├── store/
+│   │   ├── model/model.go    # GORM model structs (single source of truth)
+│   │   └── repo/             # Data Access Layer (Repository pattern, GORM)
+│   │       ├── repository.go           # Core queries, Open/OpenPostgres, AutoMigrate
+│   │       ├── repository_mutations.go # Mutation helpers (user/node/tunnel/forward CRUD)
+│   │       ├── repository_federation.go# Federation-specific queries
+│   │       ├── repository_flow.go      # Flow/forward status queries
+│   │       └── repository_control.go   # Control plane queries
 │   └── auth/                 # Auth logic
 ├── tests/                    # Integration/Contract tests
 ├── Dockerfile                # Multi-stage build (alpine)
@@ -27,23 +32,27 @@ go-backend/
 | Task | Location | Notes |
 |------|----------|-------|
 | **API Routes** | `go-backend/internal/http/router.go` | Registers handlers to `http.ServeMux` |
-| **DB Schema** | `go-backend/internal/store/sqlite/sql/schema.sql` | Embedded in binary |
-| **SQL Queries** | `go-backend/internal/store/sqlite/repository.go` | Raw SQL, no ORM |
+| **DB Models** | `go-backend/internal/store/model/model.go` | GORM structs with `TableName()` methods |
+| **Repository** | `go-backend/internal/store/repo/` | GORM-based queries, all DB ops encapsulated |
 | **Auth Middleware** | `go-backend/internal/http/middleware/jwt.go` | Extracts `Authorization` header |
 | **WebSocket** | `go-backend/internal/ws/` | Real-time updates (traffic, status) |
 
 ## CONVENTIONS
-- **No ORM**: Uses raw SQL with `database/sql` and `modernc.org/sqlite`.
-- **CGO-Free SQLite**: `modernc.org/sqlite` instead of `mattn/go-sqlite3` - builds without CGO.
+- **GORM ORM**: Uses GORM with `glebarez/sqlite` (CGO-free) and `gorm.io/driver/postgres`.
+- **AutoMigrate**: Schema created at startup via `autoMigrateAll()` — no hand-written DDL.
+- **TableName()**: All models define explicit `TableName()` returning singular snake_case names.
+- **Repository Pattern**: Handlers never access `*gorm.DB` directly — all queries go through `repo.Repository` methods.
 - **Standard Lib**: Uses `net/http` for routing (Go 1.22+ patterns).
 - **Auth**: Expects raw JWT in `Authorization` header (no `Bearer` prefix).
 - **API Envelope**: All responses use `response.R{code, msg, data, ts}` structure.
 - **Config**: Loaded from environment variables (see `cmd/paneld/main.go`).
-- **SQL Idempotency**: Prefer `ON CONFLICT DO NOTHING` for inserts in migrations/sync.
+- **SQLite Constraints**: `MaxOpenConns(1)`, WAL mode, busy_timeout=5000.
 
 ## ANTI-PATTERNS
-- **DO NOT USE** ORM - uses raw SQL throughout.
+- **DO NOT** let handlers call `repo.DB()` directly — add a Repository method instead.
 - **DO NOT CHANGE** handler signatures without updating `router.go`.
+- **DO NOT** use `type:jsonb` or `type:serial` in GORM tags (SQLite incompatible).
+- **DO NOT** omit `TableName()` on new models — GORM pluralizes by default.
 
 ## COMMANDS
 ```bash