feat: add pagination to GET /api/streams/:id/history - #900

Summary

Adds pagination to GET /api/streams/:id/history as described in #726. The endpoint now accepts page and limit query parameters (with a hard cap of 100), returns events sorted ascending by timestamp (oldest first), and responds with { data, total, page, limit }. Invalid parameters return a 400 VALIDATION_ERROR instead of being silently clamped.

Closes #726

Changes

Backend (backend/src/index.ts)

• Parse & validate query params via the shared Zod listEventsQuerySchema, matching the /api/events and /api/metrics/history patterns (coerce to int, page >= 1, 1 <= limit <= 100 → 400 with VALIDATION_ERROR on failure).
• Default limit is now 50 (was a clamped pageSize default of 20).
• History is served oldest-first (getStreamHistory(..., 'asc')).
• Response is now { data, total, page, limit }; the previous pageSize/hasMore fields are removed.
• pageSize remains a supported alias for limit for backwards compatibility.

Docs

• backend/src/swagger.ts: documented the page/limit query params, the 400 response, and the total/page/limit fields in the 200 schema.
• backend/TESTING.md: updated the manual test checklist.

Tests (backend/src/integration.test.ts)

• Updated existing pagination tests to the new response shape (limit, no hasMore) and ascending order.
• Added coverage for:
  • pagination correctness on a stream with 500+ events (disjoint slices, correct last-page remainder, ascending order);
  • 400 when limit exceeds 100;
  • 400 for invalid page/limit (zero, negative, non-integer);
  • default limit of 50;
  • pageSize alias.

Necessary build fix (backend/src/services/streamStore.ts)

• Removed two stray lines in createStream (const built = await rpcServer.prepareTransaction() left over from the Soroban wiring merge (feat(backend): wire Soroban create_stream contract call with simulati… #650) that produced a SyntaxError, preventing the module — and therefore the whole backend and its test suite — from loading. Without this the endpoint cannot even be tested. Minimal, behavior-preserving.

Acceptance criteria

• ✅ Pagination correct for streams with 500+ events — covered by the new integration test.
• ✅ Response time < 150ms — page queries use indexed LIMIT/OFFSET against stream_events (~1–10ms in test logs).
• ✅ Invalid params return 400 — covered by new tests.

Verification

• npx vitest run src/integration.test.ts — pagination/history tests pass.
• npx eslint on changed files — clean.
• ⚠️ Note: main currently has pre-existing breakage unrelated to this PR (other integration tests and the full tsc typecheck fail due to missing db.ts exports like getAllowedAssets/searchStreamsFts, Soroban claimable/reconcile wiring, and several test files). That is out of scope and intentionally left for separate follow-ups; this PR is scoped to the pagination feature.