chore: remove committed binaries and scratch dirs; tighten .gitignore

- Remove committed Go binaries:
    backend/bin/api-server (~18 MB)
    backend/cmd (~18 MB)
    backend/api/rest/cmd/api-server (~18 MB)
- Remove scratch / build output dirs from the repo:
    out/, cache/, test-results/
- Extend .gitignore to cover these paths plus playwright-report/
  and coverage/ so they don't drift back in.

Total artifact weight removed: ~54 MB of binaries + small scratch files.

.gitignore

@@ -49,3 +49,16 @@ temp/
 *.test
 *.out
 go.work
+
+# Compiled Go binaries (built artifacts, not source)
+backend/bin/
+backend/api/rest/cmd/api-server
+backend/cmd
+
+# Tooling / scratch directories
+out/
+cache/
+test-results/
+playwright-report/
+.playwright/
+coverage/

cache/solidity-files-cache.json

@@ -1 +0,0 @@
-{"_format":"","paths":{"artifacts":"out","build_infos":"out/build-info","sources":"src","tests":"test","scripts":"script","libraries":["lib"]},"files":{"src/MockLinkToken.sol":{"lastModificationDate":1766627085971,"contentHash":"214a217166cb0af1","interfaceReprHash":null,"sourceName":"src/MockLinkToken.sol","imports":[],"versionRequirement":"^0.8.19","artifacts":{"MockLinkToken":{"0.8.24":{"default":{"path":"MockLinkToken.sol/MockLinkToken.json","build_id":"0c2d00d4aa6f8027"}}}},"seenByCompiler":true}},"builds":["0c2d00d4aa6f8027"],"profiles":{"default":{"solc":{"optimizer":{"enabled":false,"runs":200},"metadata":{"useLiteralContent":false,"bytecodeHash":"ipfs","appendCBOR":true},"outputSelection":{"*":{"*":["abi","evm.bytecode.object","evm.bytecode.sourceMap","evm.bytecode.linkReferences","evm.deployedBytecode.object","evm.deployedBytecode.sourceMap","evm.deployedBytecode.linkReferences","evm.deployedBytecode.immutableReferences","evm.methodIdentifiers","metadata"]}},"evmVersion":"prague","viaIR":false,"libraries":{}},"vyper":{"evmVersion":"prague","outputSelection":{"*":{"*":["abi","evm.bytecode","evm.deployedBytecode"]}}}}},"preprocessed":false,"mocks":[]}

out/build-info/0c2d00d4aa6f8027.json

@@ -1 +0,0 @@
-{"id":"0c2d00d4aa6f8027","source_id_to_path":{"0":"src/MockLinkToken.sol"},"language":"Solidity"}

test-results/.last-run.json

@@ -1,4 +0,0 @@
-{
-  "status": "passed",
-  "failedTests": []
-}