fix: prevent splash screen after log rotation

[K0lin]

Problem

After log rotation, the splash screen would incorrectly appear even though the database already contained data. This happened because:

1. Log rotation creates a new empty file → fileSize = 0
2. Percentage calculation returned 0% (division by zero guard)
3. Frontend saw 0% progress → showed splash screen
   Additionally, hasExistingData was only set when sources existed at startup, causing issues when sources were added later.

Fix #63

Solution

• stats.go: Fix percentage calculation - return 100% when fileSize = 0 but LastPosition > 0 (data was processed before rotation)
• http_request.go: Add HasExistingData() method with caching
• coordinator.go: Move hasExistingData check to NewCoordinator() so it's always initialized before any processors are created
• processor.go: Update isInitialLoad logic to consider hasExistingData

Changes

File	Change
internal/database/repositories/stats.go	Fix percentage for post-rotation files
internal/database/repositories/http_request.go	Add HasExistingData() with caching
internal/ingestion/coordinator.go	Initialize hasExistingData in constructor
internal/ingestion/processor.go	Add hasExistingData parameter

Impact

• Splash screen now only shows on fresh installations
• No changes to initial load behavior for new deployments
• Minimal, backward-compatible changes