feat: add category support for RSS articles

[weronikakombat]

Closes #10

This PR adds support for storing and filtering articles by categories/tags from RSS/Atom feeds.

Changes

Model (internal/model/model.go)

• Added Categories []string field to Article struct

RSS Parser (internal/rss/rss.go)

• Added Categories []string to FeedArticle struct
• Capture item.Categories from gofeed when parsing feeds

Storage (internal/storage/database.go)

• Added categories column to articles table in schema
• Added migrate() function to handle existing databases (backward compatible)
• Added categoriesToString() and categoriesFromString() helpers
• Updated all article queries to include categories
• ListArticles() now accepts optional category filter parameter

Scanner (internal/scanner/scanner.go)

• Pass categories from RSS to model articles in convertFeedArticles()

Controller (internal/controller/controller.go)

• Updated GetArticles() to accept category parameter
• Pass category filter to storage layer

CLI (internal/cli/commands.go)

• Added --category / -c flag to articles command
• Display categories in article list output when present

Usage Examples

# Show all articles
blogwatcher articles --all

# Filter by category
blogwatcher articles --category "AI"

# Combined with other filters
blogwatcher articles --blog "TechCrunch" --category "Security"

Backward Compatibility

• Existing databases will be automatically migrated via migrate() function
• If categories column doesn't exist, it will be added on next database open
• Empty categories are handled gracefully (nil/empty slice)

Testing

All existing tests pass. The PR touches:

• ✅ internal/model/model.go
• ✅ internal/rss/rss.go
• ✅ internal/storage/database.go + tests
• ✅ internal/scanner/scanner.go + tests
• ✅ internal/controller/controller.go + tests
• ✅ internal/cli/commands.go

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I'm happy to address any feedback!