feat: structured startup log for loaded config summary

[extolkom]

closes #194

Summary

Adds a structured startup log that summarizes the loaded runtime configuration —
the environment context and key feature flags — emitted once when the server boots.

Previously the startup config log dumped the entire masked env config as a flat
object. This replaces it with a curated, grouped summary that is easier for
operators to scan, while guaranteeing no secrets are logged.

What changed

• New buildStartupConfigSummary() (src/utils/config-summary.utils.ts) —
  returns a grouped { environment, featureFlags } object:
  • environment: mode, port, apiVersion, stellarNetwork, backendUrl, frontendUrl
  • featureFlags: the key ENABLE_* toggles + ownership-snapshot cleanup flag
• src/server.ts — logs the structured summary at startup
  (logger.info(buildStartupConfigSummary(), 'Loaded runtime configuration summary'))
  instead of the flat full-config dump.
• Tests (src/utils/config-summary.utils.test.ts) — assert the env + flag
  values, that only those two groups are exposed, and that no sensitive values
  leak into the output.
• Docs (docs/configuration.md) — new "Startup Configuration Summary" section
  with example output and local-validation steps.

No secrets logged

Values are sourced through the existing maskSensitiveConfigValues() helper, and
only non-sensitive keys are selected. Secrets, passwords, API keys, tokens, and
the DATABASE_URL credentials are never included in the summary.

Example output

{
  "level": 30,
  "msg": "Loaded runtime configuration summary",
  "environment": {
    "mode": "development",
    "port": 3000,
    "apiVersion": "1.0.0",
    "stellarNetwork": "testnet",
    "backendUrl": "http://localhost:3000",
    "frontendUrl": "http://localhost:5173"
  },
  "featureFlags": {
    "responseTiming": true,
    "apiVersionHeader": true,
    "schemaVersionHeader": true,
    "requestLogging": true,
    "indexerDedupe": true,
    "indexerDlq": true,
    "indexerCursorStalenessWarning": true,
    "ownershipSnapshotCleanup": false
  }
}
How to validate locally
pnpm install && pnpm exec prisma generate
pnpm dev
Look for the Loaded runtime configuration summary line in the startup output and confirm no secret values appear.
Testing
pnpm jest src/utils/config-summary.utils.test.ts → passing
pnpm build (tsc) → clean
pnpm exec eslint on changed files → clean
Scope
Limited to the config-summary feature: 2 new files (util + test), 2 modified
(server.ts, docs/configuration.md). No unrelated file churn.
closes #194