Fix FreshRSS Reader API item discovery window based on published-date ot

[mncaudill]

Reader API sync for the FreshRSS/Miniflux path used lastArticleFetchStartTime to compute the ot parameter for /stream/items/ids.

On these feed readers, ot is applied to entry published date (not read/unread state-change time). This can drop entries from discovery when:

1. an entry is published before desktop’s last fetch timestamp,
2. the entry is read on phone before desktop ingests it,
3. desktop sync then queries with ot=lastArticleFetchStartTime.

Result: the item may never be fetched locally and can appear to “disappear” until manually toggled unread server-side.

Repro:

• FreshRSS/Miniflux Reader API account.
• Read an unread article on phone first.
• Open/sync desktop NetNewsWire.
• Article is missing unless marked unread again on the server.

Fix:

• For ReaderAPI variant .freshRSS, stop using lastArticleFetchStartTime for account-level item ID discovery.
• Use a stable rolling baseline window (now - 3 months) for ot instead.
• Keep existing last-fetch behavior for non-FreshRSS Reader API variants.

Tests:

• Add ReaderAPICaller tests asserting:
  • FreshRSS uses baseline window for ot.
  • Generic Reader API continues using lastArticleFetchStartTime.