Add lastFeeRound to Transcoder for independent fee factor fallback

Nobody has claimed this yet.

## 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.

Start with the Transcoder schema, the ticketBroker fee handler, and the roundsManager newRound handler described in the issue. Trace how lastRewardRound and cumulative factors are currently populated, then verify that reward and fee fallbacks use their independent rounds and that the Transcoder records the fee round.

Written by the indexing model from the issue text.