Remove KV store

[sekulicd]

We currently maintain two storage backends (SQLite and Badger), which increases maintenance overhead. Tests also run against both, making them slower.

[arkanaai]

Codebase Context

The KV (Badger) store lives in store/kv/ — five files implementing the four core store interfaces (VtxoStore, UtxoStore, TransactionStore, AssetStore) backed by dgraph-io/badger/v4 via the badgerhold wrapper:

• store/kv/utxo_repository.go
• store/kv/vtxo_repository.go
• store/kv/transaction_repository.go
• store/kv/asset_repository.go
• store/kv/utils.go (initialises 4 separate Badger DBs in subdirectories)

The SQLite backend (store/sql/) is the one to keep — it has ACID guarantees, proper migrations (store/sql/migration/), and generated query code via sqlc.

Callers that need updating:

• types/types.go — remove the KVStore = "kv" constant
• store/service.go — remove the case types.KVStore branch and the kvstore import
• client.go (lines 76, 140) — default AppDataStoreType is currently types.KVStore; switch to types.SQLStore
• store/service_test.go — test matrix iterates over both kv and sql; remove the kv entry

Dependencies to drop from go.mod:

• github.com/dgraph-io/badger/v4
• github.com/timshannon/badgerhold/v4

Acceptance Criteria

• store/kv/ package deleted
• KVStore constant removed from types/types.go
• store/service.go no longer references the kv package or KVStore case
• Default store in client.go changed to SQLStore
• Test matrix in store/service_test.go reduced to SQL-only; test times improve
• badger and badgerhold removed from go.mod / go.sum
• go mod tidy passes cleanly
• All existing store tests pass against SQLite

Suggested Approach

1. Delete store/kv/
2. Update types/types.go, store/service.go, client.go as above
3. Trim the test matrix in service_test.go
4. Run go mod tidy to drop unused deps
5. Verify with go test ./store/...

No interface changes needed — SQLite already satisfies all four store interfaces.