Migrate to Static Backend for Mentors and FAQ

[ppeelen]

This PR completes the migration from Firebase to a static file-based backend system for all conference content that doesn't change frequently during the event.

Changes Made

Infrastructure (@nvh)

• Added DataSync class and FileStore system for fetching static content from GitHub
• Implements ETag-based caching for efficient content updates
• Provides atomic file operations and persistent local storage
• Added environment variable support for branch selection via SWIFTISLAND_BRANCH

Complete Data Migration (@ppeelen)

• Mentors → api/mentors.json (already migrated by @nvh)
• FAQ Items → api/faq.json
• Locations → api/locations.json
• Pages → api/pages.json
• Activities → api/activities.json
• Events → api/events.json
• Packing Items → api/packing-items.json
• Puzzles → api/puzzles.json

Enhanced Configuration

• Branch-configurable content fetching using SWIFTISLAND_BRANCH environment variable
• Defaults to main branch if no environment variable is set
• Enables testing with different data sets on feature branches

Technical Details

Static Content URLs:

• Default: https://raw.githubusercontent.com/SwiftIsland/app/refs/heads/main/api/*.json
• Configurable: https://raw.githubusercontent.com/SwiftIsland/app/refs/heads/{BRANCH}/api/*.json

Key Features:

• ETag-based HTTP caching prevents unnecessary downloads
• Graceful fallback to cached content when network is unavailable
• Consistent error handling across all data types
• Maintains all existing data relationships (Events → Activities, Activities → Mentors)
• Preserves sorting and filtering logic

Functions Migrated:

• fetchLocations(), fetchPages(), fetchActivities(), fetchEvents()
• fetchPackingListItemsFromFirebase(), fetchPuzzles(), fetchFAQItems()

Functions Unchanged:

• fetchTicket(), fetchAnswers() (External Tito API)
• fetchSponsors() (Already uses external URL)

Migration Impact

This is a breaking change for deployments that rely on Firebase for content data. The app will now fetch all conference content from static JSON files in the repository instead of Firebase collections.

Static JSON files are provided in /api/ directory with example data that matches the existing entity models.