Implement API quality & reliability improvements

[katefominmoore23]

• Confirm dead-letter queue requeue endpoint enforces idempotency for double-requeue #1519: Dead-letter queue requeue idempotency enforcement

  • RequeueResult enum distinguishes NotFound vs NotInDeadLetter
  • Handler returns 409 CONFLICT for status mismatch (not silent success)
  • Data layer idempotent independent of HTTP cache TTL
  • Test confirms no duplicate sends on double-requeue

• Add contract test ensuring handlers.rs ApiError variants map to documented OpenAPI error schemas #1520: API error variants to OpenAPI schema mapping contract test

  • Enumerates all ApiError variants and required schemas
  • Contract test fails if new variant added without documentation
  • Prevents undocumented error responses to clients
  • Current coverage: 6 error variants (400/404/409/429/500/503)

• Bound in-memory watched_txs restore on startup for very large tx-watch tables #1521: Bounded in-memory watched_txs restore on startup

  • Max restore limit: 50,000 transactions
  • Warning logged when table exceeds bound
  • Startup metric tracks watched_tx_count
  • Prevents memory exhaustion on large tables

• Add negative-path tests for pagination.rs offset+cursor combined usage #1522: Pagination mutual exclusivity (offset vs cursor)

  • Validates offset and cursor are mutually exclusive
  • Returns 400 BAD_REQUEST with clear error message
  • Unit tests cover all four parameter combinations
  • API docs clarify pagination strategy separation

Closes #1519
Closes #1520
Closes #1521
Closes #1522

Claude-Session: https://claude.ai/code/session_014jDDop7frnew1xcCJDSKEw

Description

Type of Change

• Bug fix
• New feature
• Refactor / code cleanup
• Documentation update
• CI / tooling change
• Breaking change

Testing Done

Bundle Size

Chunk	Before	After
vendor.js
main*.js
pages/_app*.js

Checklist

• Tests pass locally
• Documentation updated (if applicable)
• No breaking changes, or breaking changes are documented above
• If you added or changed an API endpoint, regenerated the OpenAPI spec and committed the result:

  cd services/api && cargo run --bin generate-openapi > openapi.yaml
  git add openapi.yaml && git commit -m "chore: regenerate openapi.yaml"

• If you changed system architecture (new service, database, external dependency, or network boundary), updated docs/architecture.md
• Bundle size checked (if frontend changes)

Related Issues

Closes #

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