refactor: improve storage.ts cohesion by extracting modules

[coji]

Problem

storage.ts is 1083 lines with mixed concerns: type definitions, pure data transformers, backend-specific logic (SQLite vs PostgreSQL), and cross-cutting mutex infrastructure.

Extraction Opportunities

1. Data transformers → transformers.ts

• validateLabels(), rowToRun(), rowToStep(), rowToLog(), toClientRun()
• Pure functions, zero DB dependency, ~80 lines
• Testable in isolation

2. claimNext backend split

• claimNext() is 140+ lines with two completely divergent paths:
  • PostgreSQL: FOR UPDATE SKIP LOCKED + advisory locks + retry loop (lines 716–799)
  • SQLite: simple subquery + UPDATE (lines 801–836)
• Could become claim-sqlite.ts and claim-postgres.ts
• Biggest readability win

3. Mutex → keep inline (too small to extract)

• createWriteMutex() is 18 lines, only used once
• Not worth a separate file

Priority

Medium. Do alongside #124 (getRun JOIN optimization) since both touch the same file.

References

Found during /simplify cohesion review on PR #130