Refactor to eliminate code duplication and improve maintainability

[powerje]

Summary

This PR implements a comprehensive refactoring to eliminate code duplication, improve maintainability, and enhance testability across the codebase. The changes eliminate ~315 lines of duplicated code while adding ~621 net lines (including extensive test coverage).

Key Changes

Code Duplication Elimination (~315 lines removed):

• Extract server URL retry logic to reusable TryServerURLs helper (~75 lines)
• Add WithUser middleware to eliminate repeated user DB fetches (~130 lines)
• Extract main.go initialization logic to app.Initializer (~110 lines)

New Components:

• internal/helpers/plex.go - Generic helper for trying multiple server URLs with fallback
• internal/middleware/user.go - Middleware to fetch user once and add to context
• internal/app/initializer.go - Extracted initialization logic from main.go

Bug Fixes:

• Fix context.Background() usage in pairing handler with proper 30-second timeout

Infrastructure:

• Add database transaction support (BeginTx, WithTransaction helper)
• Comprehensive test coverage for all new functionality

Files Changed (18 files)

• New files: internal/helpers/plex.go, internal/helpers/plex_test.go, internal/middleware/user.go, internal/middleware/user_test.go, internal/app/initializer.go
• Modified: main.go, internal/db/db.go, internal/db/db_test.go, handlers (media.go, mappings.go, pairing.go, play.go, media_detail.go), handler tests

Commits (8)

1. Add TryServerURLs helper to eliminate server retry duplication
2. Refactor media.go to use TryServerURLs helper
3. Refactor mappings.go to use TryServerURLs helper
4. Add WithUser middleware to eliminate user fetch duplication
5. Integrate WithUser middleware into auth chain
6. Add WithUser middleware and refactor handlers to use user from context
7. Fix context.Background() usage in pairing handler with timeout
8. Add transaction support and extract main initialization logic

Test Plan

• All existing tests pass (verified with ./checks.sh)
• New test coverage added:
  • internal/helpers/plex_test.go - TryServerURLs helper tests
  • internal/middleware/user_test.go - WithUser middleware tests
  • internal/db/db_test.go - Transaction tests (BeginTx, WithTransaction)
• Code formatting passes (go fmt)
• Linting passes (golangci-lint - 0 issues)
• Race detector passes (go test -race)
• Build succeeds

Impact

• Lines changed: +960 insertions, -339 deletions (net +621, including tests)
• Code quality: Eliminated significant duplication, improved separation of concerns
• Maintainability: Extracted reusable components, better testability
• No breaking changes: All existing functionality preserved