Add lastFeeRound to Transcoder for independent fee factor fallback

[rickstaa]

Problem

The contract's latestCumulativeFactorsPool() tracks reward and fee factors independently using separate lastRewardRound and lastFeeRound fields. The subgraph only has lastRewardRound on the Transcoder entity.

When a transcoder reactivates after being inactive, the roundsManager newRound handler falls back to lastRewardRound for both cumulativeRewardFactor and cumulativeFeeFactor. If fees were earned in a later round than the last reward (i.e. lastFeeRound > lastRewardRound), the propagated cumulativeFeeFactor will be stale.

Example

• Round 98: reward() called, fees earned → lastRewardRound=98, lastFeeRound=98
• Round 99: no reward(), fees earned → lastRewardRound=98, lastFeeRound=99
• Rounds 100-104: deactivated
• Round 105: reactivated → subgraph falls back to round 98 for both factors, missing the fee factor update from round 99

Fix

1. Add lastFeeRound: Round to the Transcoder schema
2. Set transcoder.lastFeeRound = round.id in the ticketBroker fee handler
3. In roundsManager newRound, use separate fallback lookups:
   • cumulativeRewardFactor from lastRewardRound pool
   • cumulativeFeeFactor from lastFeeRound pool

This mirrors the contract's independent lastRewardRound/lastFeeRound lookups in latestCumulativeFactorsPool().

Context

Related to PR #217. The reactivation fallback to lastRewardRound was added in that PR but only covers the reward factor correctly.