feat(backend): implement SorobanVaultChainInvoker — wire on-chain vault pause/unpause

[Salmatcre8]

Summary

• Adds internal/stellar package with ContractInvoker: builds and submits InvokeHostFunction transactions to a Soroban RPC node using simulate → patch soroban data → sign → send → poll flow
• Adds SorobanVaultChainInvoker in internal/service that implements VaultChainInvoker by calling pause/unpause on the on-chain vault contract
• Wires the real invoker in main.go when STELLAR_OPERATOR_SECRET is set; falls back to NoopVaultChainInvoker when it is absent (preserves backward-compat for envs without a configured operator key)
• Adds STELLAR_OPERATOR_SECRET optional config field (loaded from env, no validation — empty is valid)

How it works

AdminService.PauseVault
  → SorobanVaultChainInvoker.PauseVault(ctx, contractAddress)
    → ContractInvoker.InvokeVoidFunction(ctx, contractAddress, "pause")
      1. GET /accounts/{operator} from Horizon → sequence number
      2. Build InvokeHostFunction tx (caller = operator keypair)
      3. POST simulateTransaction → get SorobanTransactionData + minResourceFee
      4. Patch envelope with soroban data and bumped fee
      5. Sign with operator keypair
      6. POST sendTransaction
      7. Poll getTransaction until SUCCESS / FAILED
  → repository.UpdateVaultStatus(ctx, id, StatusPaused)

Test plan

• go test ./internal/stellar/... — 7 unit tests pass (address encoding, simulate-error path, send-error path)
• go test ./internal/service/... — invoker construction, invalid-contract rejection, RPC error propagation, noop default
• GOOS=linux go build ./... — clean Linux build

Closes #242

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[0xDeon]

Lgtm