[Feature] Better Feeds Tagging System

Current labeling and user query system allow user to do things like below:

- **Search & filter feeds** based on custom filters
- Automatically **add label new articles of a feed** by custom filters
- **Mark new articles as read** by custom filters and other settings

However, **things become quite hard when we try implementing "tagging" logical structure using FreshRSS label system**. Let's see an example below:

# Implement Feed Tagging by Label

## Initialization

Consider our logical tag structure is as below:

| FeedID | Feed | Logical Tags |
| ------ | ----------------------------- | -------------------- |
| 1 | FreshRSS GitHub Issue | `github`, `selfhost` |
| 2 | GitHub Dependencies Alerts | `github`, `notice` |
| 3 | Selfhost Server States Update | `selfhost`, `notice` |

The most intuitive way to achieve such "tagging" structure in current version of FreshRSS is to use **label system**.

| Label Name | Label Filters |
| ---------- | ------------- |
| `github` | `f:1,2` |
| `selfhost` | `f:1,3` |
| `notice` | `f:2,3` |

Looks good until we start considering following cases.

## Add / Remove Multiple Tags to / from Feed

Consider we need to add a new feed "FreshRSS Security Updates" *(let's say its feed id is `4`)*, which need to be tagged as `github`, `selfhost` and `notice`.

The **complexity goes up if there are lots of "tags" we want to add to a new feed**. In case above, we need to go to the settings of 3 different labels and update their filters expression to add `4` to the feed id filter.

| Label Name | Label Filters |
| ---------- | ------------- |
| `github` | `f:1,2,4` |
| `selfhost` | `f:1,3,4` |
| `notice` | `f:2,3,4` |

Similar operations required when we want to remove multiple tags from a feed. Also, it would be a disaster if I somehow need to delete and re-add a feed which has lots of tags.

# My Opinion

## About the ID Filter

Personally, I think **it's not a very good idea to allow users to hardcoded the unique id of the feeds/categories/labels into the filter expression** which will be stored into the FreshRSS system to achieve some automation work. Two major reasons for this:

- Not quite elegant.
- **Users are generally more familiar with the "name" of the feeds/categories/labels than IDs of them.** So we user need to filter by feeds/categories, since there is no "filter by feed name/filter by category name" filter, user need to first work out all IDs of relevant feeds/categories before writing filter expression.
- It could be a more elegant and more convenient approaches to allow users to select these instances. Like a text input UI with dropdown multi-selection & auto-completion based on the name of those feeds/categories/labels. *(Though, I know this may not feasible consider the high complexity of the filter system)*
- IDs could expire.
- If those feeds/categories/labels got deleted. In this case, there's no sense that their ID still appears in the filter expressions *(even if these "dummy" id may not affect the actual filter result of the filter, it's just not elegant)*. It's like require user to manage something like "foreign key" manually.
- Another way to address this may be some "cascade deletion" feature. For example, when a feed with id `x` is about to be deleted, system will check all filter expression used in user query/label filter etc. and inform user "Corresponding Feed ID will be removed in following filters: ..." and automatically remove those unused ID in filter expressions in the system.

## About How Labeling Works

The major inconvenient, as illustrated in the above example, is the complexity of the operations. **If we need to modify `n` tags of a feed, we need to update `n` different query expression in `n` different label settings.**

One better way may be to allow user to operate in another direction: Allow user to specify target label/tag in feed settings.

For example, there could be some settings for feed like:

- **Add following tags to all articles of this feed**: (a list separated by comma or something, like: `github,issue,selfhost`)
- Add following label to all articles of this feed: (a list separated by comma or something, like `label1,another_label_name`, label ID may also be allowed here, like `1,3,4,7`)

By adding this design, we only need to modify settings in one place when we want to edit the tags of a feed.

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Above are just some of my own personal thought and there could be some better approaches to solve such issue. I believe tag-like structure is really a common use case and there should be a convenient way for users to achieve that.