fix: preserve RSS history across feed rollover

[vycdev2]

Summary

• detect existing RSS feed history independently of the entries in the current response
• prevent rotating feeds from being mistaken for first-time subscriptions and dropping all but the latest new item
• add a SQLite regression test for history that has rolled out of the current feed

Verification

• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --filter FullyQualifiedName~RssFeedJobTests — passed: 4/4 tests
• dotnet build — passed: 0 errors; 2 existing NU1903 warnings for SQLitePCLRaw.lib.e_sqlite3 2.1.6
• dotnet test — passed: 227/227 tests
• git diff --check upstream/develop...HEAD — passed

Risk

• Low: the change adds one indexed existence query per subscribed feed and leaves initial-feed backfill behavior unchanged.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.