Add `--json` output option to CLI list commands

[fuleinist]

Problem

CLI commands like list feeds and list entries output plain text, making it difficult to parse results in scripts or pipe to other tools.

Proposed Solution

Add a --json flag to the list subcommands that outputs structured JSON (one JSON object per line for easy streaming/processing).

Use Case

Users building automation scripts can easily parse feed/entry data with tools like jq or in any programming language. For example:

reader list feeds --json | jq '.url'
reader list entries --json | jq -r '"\(.feed_url) \(.title)"'

This would enable better integration with monitoring tools, custom dashboards, and CI/CD pipelines.

[lemon24]

Hi, I am OK with having this as a feature, and am willing to review a PR for it.

Some suggestions:

• consider using either something like _hash_utils._dataclass_dict or cattrs (fewer dependencies is better)
• the output should be JSON lines, not an array of JSON objects (can't easily generate streaming JSON)

(If you haven't already, please have a look at the contribution guidelines, including the newly-added AI policy.)

Thank you!