Subgraph Audit and Pre-Feature Performance Improvements

## Problem

The subgraph has grown and BuilderDAO flagged performance and cleanliness improvements while merging earlier work (PRs #168 and #175). Before adding the stake and earnings feature (#238, implemented in PR #217), we should audit the subgraph to identify what needs improving and in what priority, so new features are built on a clean and performant base rather than compounding existing issues.

## Scope

- Review current mappings, schema, and indexing for performance and maintainability issues
- Consolidate the improvements flagged in PRs #168 and #175
- Assess the impact of the upcoming stake and earnings feature (#238, PR #217) on subgraph size and indexing performance, using the tooling from #239
- Produce a prioritized backlog that separates blocking work (must precede the feature), high value improvements, and deferred cleanup

## Deliverables

- A written audit summary of the subgraph's current state
- A prioritized improvement backlog captured as linked issues
- A clear recommendation on which fixes must land before #238 and which are deferred to the performance and cleanup phase

## Definition of done

The audit summary is published and the prioritized backlog exists as issues, with any blocking items clearly flagged.

## Related

- Measurement tooling: #239
- Feature this precedes: #238 (PR #217)
- Known improvements to fold in: #180, plus the items flagged in PRs #168 and #175

## Sequence

Audit and tooling (this issue and #239), then stake and earnings indexing (#238, PR #217), then performance and cleanup (#180 and the remaining flagged items).

Start by reviewing the subgraph's current mappings, schema, and indexing, then read the improvements flagged in PRs #168 and #175 and the measurement tooling in #239. Done means publishing an audit summary, creating a prioritized backlog as linked issues, and clearly flagging work that must precede #238.

Written by the indexing model from the issue text.