docs: add scheduled task concurrency and process lifecycle patterns

[moltboie]

Add two new design pattern sections to DEVELOPMENT.md:

Scheduled Task Concurrency

Documents the concurrency guard pattern for long-running scheduled tasks (like scheduledSync). Without a guard, slow API responses can cause overlapping executions that race on shared data.

Related: #222

Process Lifecycle Handlers

Documents that process-level signal handlers (SIGINT, SIGTERM, unhandledRejection, uncaughtException) should live in start.ts, not in library modules. Library imports shouldn't have global side effects.

Related: #223

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E2E: Verified DEVELOPMENT.md renders correctly and sections are properly ordered within the Design Patterns section.

[moltboie]

Self-Review

Discussion thread status:

• New PR. Documentation-only: adds two DEVELOPMENT.md patterns from recent PRs (reliability: scheduled sync has no concurrency guard #222 scheduled sync concurrency, refactor: move process error handlers from postgres/client.ts to start.ts #223 process lifecycle handlers).

Checked:

• Logic: Patterns accurately reflect the implementation in PRs reliability: scheduled sync has no concurrency guard #222/refactor: move process error handlers from postgres/client.ts to start.ts #223. Code examples are correct TypeScript. ✓
• Types: Docs only — N/A.
• Quality: Sections placed appropriately in Design Patterns. Clear before/after examples. ✓
• Maintainability: Future contributors will have explicit guidance on these patterns. ✓
• Security: No code changes. ✓
• Tests: N/A for documentation.
• Blockers: None.

E2E Testing:

• Documentation change only. Verified DEVELOPMENT.md renders correctly and the two new sections appear under Design Patterns in proper order.

Issues found:

• None.

Confidence: High

[moltboie]

Rebased on main.