#114 [High] Add a load-testing pipeline exercising intent creation, quoting, and accept flows

[james2177]

Description: There is no load/performance testing anywhere in the repository — npm test/npm run test:e2e validate correctness, not throughput or behavior under concurrency, despite known concurrency-sensitive code paths already flagged elsewhere in this backlog (issue #65's idempotency-key race, issue #76's check-then-act race in update()). Add a load-testing suite (k6, Artillery, or similar) targeting POST /api/v1/intents, the quote endpoint, and the accept endpoint under realistic concurrent load.

Problem Statement & Context: Races like the ones in issues #65 and #76 are exactly the class of bug that only reliably surfaces under real concurrent load — a load-testing pipeline both validates fixes for those issues and establishes a performance baseline before the CD/staging pipelines in issues #28/#110 start shipping regularly.

Scope & Acceptance Criteria:

• Add a load-test suite (new load-tests/ directory or similar) covering: concurrent intent creation, concurrent quote requests, and concurrent accept-on-the-same-intent (the exact race class from issues Guard BigInt(dto.fillAmount) against malformed input #65/Add idempotency keys to intent creation #76) against a running instance.
• Add a CI job (manually triggered or scheduled, not on every PR given runtime cost) that runs the suite against the staging environment from issue Add test coverage reporting and a minimum threshold in CI #110 and reports latency/error-rate results.
• Out of scope: fixing any races the load tests uncover — file those as new issues; this issue delivers the testing tooling and baseline report.
• Explicitly out of scope: load-testing on-chain Soroban RPC calls directly against a real network (use the dry-run flag from issue Add secrets scanning to CI #51 or mocked responses to avoid hammering real infrastructure).

Implementation Guidelines:

1. Key files/endpoints: src/intents/intents.controller.ts, src/intents/intents.service.ts (the create()/update() methods referenced by issues Guard BigInt(dto.fillAmount) against malformed input #65/Add idempotency keys to intent creation #76), src/solvers/solvers.controller.ts.
2. Model concurrent-accept scenarios explicitly (multiple simulated solvers racing to accept the same open intent) since that's the highest-value scenario for validating issue Add idempotency keys to intent creation #76's fix.
3. Keep test data isolated (a dedicated load-test user/solver namespace) so results don't pollute real data in whatever environment it targets.
4. Testing: the load-test suite itself needs to be run and its output (latency percentiles, error rate, and specifically whether any double-accept/race condition was observed) included as evidence in this issue's PR.

Definition of Done:

• Load-test suite implemented, CI job added, baseline report produced.
• Acceptance criteria met.
• PR passes CI, includes load-test run output/report.
• Reviewed and approved.

Resources: src/intents/intents.controller.ts, src/intents/intents.service.ts, src/solvers/solvers.controller.ts

Complexity: High (200 points)