feat: error-context helper, DB pool startup log, last-page cursor test, indexer retry docs

[tosin-zoffun]

Summary

Four small, independent improvements.

#302 — Structured error context helper

Adds buildErrorContext(err, options) in src/utils/error-context.utils.ts: produces a consistent { name, message, code, requestId?, stack?, timestamp } from any caught value (Error subclasses, Prisma P* errors, Zod/JWT by name, plain strings). The error code is normalized to the shared ErrorCode values, and the stack is debug-only (omitted unless includeStack). Applied in the global error middleware's structured log so the request id and normalized code are logged together.

#298 — Startup log for DB connection pool config

describeDatabasePoolConfig() parses DATABASE_URL for the pool settings Prisma reads (connection_limit, pool_timeout, connect_timeout) plus the query-timeout, and server.ts logs them right after prisma.$connect() — before the server accepts requests. No credentials or host details are included (parsing failures degrade to 'default').

#301 — Integration test: cursor → partial last page

The existing cursor round-trip test covers an even split (6 items / two full pages). This adds creator-list-last-page.integration.test.ts for the end-of-list edge: a known total of 5 at limit=2 yields pages [2, 2, 1]; the last page returns only the remaining item and meta.hasMore=false. A second test walks every page and asserts the traversal reconstructs the full set exactly once.

#297 — Indexer retry/backoff docs (docs-only)

Adds docs/indexer/RETRY_BACKOFF.md: documents the exponential backoff with jitter (getBackoffWithJitter: base 1000ms, ×2^attempt, cap 30000ms, jittered by INDEXER_JITTER_FACTOR), the controlling config values, and the DLQ exhaustion behavior (moveToDLQ → indexerDLQ, observable via getDLQDepth/syncDLQMetrics; the risk of silent loss when the DLQ is disabled).

Test plan

• tsc --noEmit — 0 errors (after prisma generate).
• New tests pass: error-context.utils.test.ts (6), db-pool-config.utils.test.ts (4), creator-list-last-page.integration.test.ts (2).
• prettier --check + eslint clean on changed files (also enforced by the lint-staged pre-commit hook).
• Full suite: no new failures introduced — passing suites 43 → 46, passing tests 365 → 377.

Notes / out of scope

• The full jest run has 17 pre-existing failed suites / 8 failed tests on main (unrelated files, e.g. activity.service.test.ts) — confirmed identical with this change stashed; this PR introduces zero new failures. Integration tests require prisma generate first.

Closes #297
Closes #298
Closes #301
Closes #302

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[Akpolo]

Follow-up: wired the two helpers into the live code paths so they're actually exercised (the first commit only added them):

• error.middleware.ts now logs via buildErrorContext instead of ad-hoc console.error (Add helper for building structured error context from caught exceptions #302).
• server.ts logs describeDatabasePoolConfig() once at startup (Add startup log for database connection pool configuration #298).

Verified: tsc --noEmit clean, helper unit tests pass (10/10), eslint/prettier clean.