Perf: scaffold DB perf test project (manual-first)

[joeldickson]

Summary

Kick off v1 of DB performance testing scaffold from latest main.

Included in this PR

• New perf test project: src/Agoda.DevExTelemetry.DbPerfTests
  • PerfWebApplicationFactory for SQLite/PostgreSQL host boot
  • deterministic/randomized seeding (PerfDataSeeder, PerfSeedOptions)
  • warm-up + endpoint timing tests with p50/p95/p99 JSON output
  • PostgreSQL helper with env-based admin connection and testcontainer fallback
• Documentation: docs/db-perf-tests.md
• README link to perf test docs

Notes

• Manual-first approach, aligned with issue Proposal: add dedicated DB performance test project + CI benchmark job #16.
• Relative gating and baseline delta checks remain a follow-up after stabilization.
• Current guardrail is non-catastrophic p95 threshold only.

Follow-up needed (auth scope)

A workflow_dispatch workflow file is prepared locally (.github/workflows/db-perf-manual.yml), but the current token cannot push workflow files (missing workflow scope).

Once a workflow-scoped token is available, I can push that workflow immediately in a follow-up PR (or this one if preferred).

Relates to #16

[github-actions]

DB Perf Report

Run: 23448870821
Baseline: not available yet (first run or no successful main artifact found)

Engine	Endpoint	Mean ms	p95 ms	Δ p95 vs main
PostgreSql	/api/build-metrics/api-summary?environment=all	33.65	42.81	n/a
PostgreSql	/api/build-metrics/clientside-summary?environment=all	28.13	38.24	n/a
PostgreSql	/api/build-metrics?page=1&pageSize=50&environment=all	3.49	4.43	n/a
PostgreSql	/api/test-runs/summary?environment=all	30.01	56.71	n/a
PostgreSql	/api/test-runs?page=1&pageSize=50&environment=all	3.00	3.35	n/a
Sqlite	/api/build-metrics/api-summary?environment=all	43.34	52.86	n/a
Sqlite	/api/build-metrics/clientside-summary?environment=all	37.52	46.98	n/a
Sqlite	/api/build-metrics?page=1&pageSize=50&environment=all	1.85	2.26	n/a
Sqlite	/api/test-runs/summary?environment=all	34.62	40.28	n/a
Sqlite	/api/test-runs?page=1&pageSize=50&environment=all	1.47	1.70	n/a