fix: retain reminders when channels are unavailable

[vycdev2]

Summary

• Keep reminders when the Discord channel is temporarily unavailable from the client cache.
• Record the miss through the existing bounded retry/backoff policy instead of deleting the reminder immediately.
• Share retry scheduling between unavailable-channel and send-failure paths, with regression coverage.

Verification

• dotnet restore Morpheus.sln — passed; existing NU1903 SQLitePCLRaw.lib.e_sqlite3 vulnerability warning.
• dotnet build Morpheus.sln --no-restore --verbosity minimal — passed (0 errors; same NU1903 warning).
• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --no-build --filter FullyQualifiedName~RemindersJobTests --verbosity minimal — passed (10/10).
• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --no-build --verbosity minimal — 298/300 passed; two pre-existing tr-TR globalization-invariant failures in MiscModuleTests.
• git diff --check upstream/develop...HEAD — passed.
• Sabotage check: temporarily restored immediate-delete behavior; the new regression test failed, then the fix was restored and focused tests passed.

Risk

• Low: changes are limited to reminder delivery retry bookkeeping and its focused tests. No schema changes.

Closes #111

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