BOLT-910: Technical documentation for historical backfill feature

Public-facing technical doc covering how historical backfill works, per-OEM quirks (SolarEdge / Enphase / FranklinWH / Tesla / EG4), and how to use both the GraphQL mutation (triggerHistoricalBackfill) and admin REST endpoint to enable backfills and pull historical data. Draft into docs/ in mono first for review before promoting to the public docs repo. References: - adapters/oem/specs/generic-historical-data-backfill.md - specs/20260424-bolt-601-historical-data-backfill/spec.md + oem-nuances.md + issue-breakdown.md - domains/device/src/subgraph/schema/mutations/triggerHistoricalBackfill.ts - adapters/oem/src/routers/admin/backfillStatus.ts - adapters/oem/src/routers/admin.ts (POST /:manufacturer/queue/historical-data) Plan: 1. Worktree ../mono-bolt-910-historical-backfill-docs, branch artem-agent/bolt-910-historical-backfill-docs. 2. Write docs/historical-backfill.md — single comprehensive doc for review (overview, architecture, API contract for both mutation + admin REST, OEM quirks matrix, status polling, error / skip codes, examples). 3. Open draft PR linked to BOLT-910 — Artem reviews then we promote to public docs repo.