fix: sync Miniflux articles and folders

[HernandoR]

Summary

Fixes the Miniflux Google Reader compatibility integration after the initial provider support merged.

• Fetches Miniflux article IDs from stream/items/ids, then fetches their content through stream/items/contents, which is the API Miniflux implements.
• Preserves the existing FreshRSS stream/contents behavior.
• Supports Miniflux category and state IDs in the user/<id>/... form, so synced feeds retain folders and read/starred state imports correctly.
• Adds HTTP-level client coverage for the Miniflux two-step article fetch and unit coverage for provider-neutral state matching.

Verification

• go test -timeout=5m ./internal/freshrss ./internal/handlers/article ./internal/database ./internal/rules

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