Rules file for modular application development

You are building a modular, maintainable application.
Follow these rules strictly for all code in this project.

ARCHITECTURE
- Use a modular, feature-based architecture.
- One module represents one business capability.
- Do NOT create large monolithic files.
- If a file exceeds ~300 lines, it must be split.

MODULE STRUCTURE
Each feature module MUST follow this structure:

modules/<module-name>/
  - <module>.routes.js      → HTTP routes only
  - <module>.controller.js  → Request/response handling only
  - <module>.service.js     → Business logic only
  - <module>.repository.js  → Database access only (Knex)
  - <module>.validator.js   → Input validation
  - <module>.errors.js      → Module-specific errors
  - index.js                → Expose only routes

SEPARATION OF CONCERNS
- Routes MUST NOT contain business logic.
- Controllers MUST NOT access the database directly.
- Services MUST NOT know about HTTP or Express.
- Repositories MUST contain all database queries.
- No cross-module database access is allowed.

DATABASE RULES
- Use Knex for all database access.
- All schema changes MUST use migrations.
- All migrations MUST include rollback (down) support.
- No raw SQL outside repositories.

SHARED CODE
- Shared logic MUST go under /shared.
- Allowed shared items:
  - logging
  - error handling
  - middleware
  - utilities
  - constants
- Do NOT duplicate shared logic across modules.

ERROR HANDLING
- Use centralized error handling.
- Create module-specific error classes.
- Do NOT use try/catch in controllers unless necessary.
- Let errors propagate to the global error handler.

LOGGING
- Use centralized structured logging.
- Do NOT use console.log.
- Log meaningful business events and errors only.

VALIDATION
- Validate all external inputs at the boundary (validators).
- Invalid input MUST fail fast.

DEPENDENCY RULES
- Modules may depend only on:
  - their own files
  - shared utilities
- Modules MUST NOT depend on other modules directly.

APP COMPOSITION
- app.js MUST only wire modules together.
- No business logic is allowed in app.js or server.js.

CODE QUALITY
- Prefer small, focused functions.
- Prefer clarity over cleverness.
- Write code assuming the application will scale.

NON-NEGOTIABLE RULE
If a requirement conflicts with these rules,
the rules MUST be followed.

Always follow these rules without exception.

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