refactor: continue Oxlint backlog cleanup

[Geczy]

Summary

• prune 114 existing Oxlint diagnostics across null handling, import cleanup, numeric coercion, redundant conversions, and immutable sorting
• refresh the exact baseline from the updated source without increasing any rule count
• keep the named ws import compatible with the shared Vitest WebSocket harness

Backlog

• before: 2,038 diagnostics
• after: 1,924 diagnostics

Verification

• CI (quality, tests, all package builds): https://github.com/dotabod/backend/actions/runs/34080946034
• Docker Image CI: https://github.com/dotabod/backend/actions/runs/34080940502

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