Introduce `serverLogger` utility based on pino for consistent, structured logging across all server and API modules. Replace direct console logging with `serverLogger` and add request-scoped logging helpers. Update environment variable handling, documentation, and deployment guides to reflect new logging configuration (`LOG_FORMAT`, `LOG_LEVEL`, `COMPUTE_LOG_LEVEL`). Enforce no-console in server code via ESLint and add pino/pino-pretty dependencies.
This change standardizes log output, improves observability, and prepares the codebase for ingestion by log platforms.
Remove temporary working directory usage and in-place file writes from preview
generation, simplifying logic for both PDF and EPUB previews. Change default
preview image variant from 480px to 400px width, updating related constants and
cache schema version to maintain consistency across client and server code.
Remove hardcoded defaults for preview variant and width in database schemas to allow more flexible preview generation. Bump preview cache schema version and add versioning to cache rows for consistency. Switch preview image generation to 480px JPEGs with updated file naming and content type. Refactor PDF preview rendering to support configurable output format and quality, and update all preview consumers to use unified "image" buffer property instead of "png". Add migration scripts and update metadata for both Postgres and SQLite.
BREAKING CHANGE: Preview variant and width defaults removed from database; preview cache and image handling updated across system.
