[Feature] Replace or Complement Categories with a Tag-Based System for RSS Feeds

### Is your feature request related to a problem? Please describe.

Currently, FreshRSS assigns each feed to a single static category. This works well for basic organization, but it's limiting for users with diverse interests or overlapping topics. I’d like to propose a tag-based system, where feeds can be assigned multiple tags, offering a more dynamic and powerful way to organize and filter content.

### Use case

The current category model enforces a one-to-one relationship between a feed and a category. However, many feeds fit naturally into multiple topics. For example:

- A feed on climate science might be relevant to both Science and Environment
- A blog might be both Personal and Tech
- Under the current system, users have to make a choice—potentially sacrificing clarity or discoverability.

### Describe the solution you’d like

Introduce a tag-based system for feeds:

- Allow feeds to have multiple tags
- Tags can function similarly to categories in the UI, possibly with a tag-based navigation sidebar
- Tags could also be used for filtering, rules, or searching
- Existing categories could be migrated to tags (or tags could coexist with them)

### Describe alternatives you’ve considered

Smart Filtering or Search-Based Grouping
Tag-like filtering based on content, keywords, or rules (e.g. filters based on feed title) could also be used to simulate tags. However:
- This requires constant tuning of filters
- It's harder to manage and less intuitive than simply tagging a feed

### Additional context

Benefits:

- More flexible content organization
- Better discoverability of content across intersecting interests
- Easier to manage large numbers of feeds without creating an overly nested or rigid structure

Would love to hear what the maintainers and community think about this! I'm also happy to help flesh out the idea further or contribute code if needed.