feat(config_flow): per-provider reauth flows (Phase 8 PR-5)

[Artic0din]

Summary

PR-5 — First plank of HACS Silver. When Amber, LocalVolts, or OpenElectricity (DWT-OE) rejects an API key, PriceHawk raises ConfigEntryAuthFailed and HA prompts the user to re-enter credentials via the integration UI. Closes the loop that today produces silent WARNING spam (Amber / LocalVolts) or a stuck setup (DWT-OE raises but no reauth handler picks it up).

• coordinator.py: Amber _fetch_amber_with_retry raises on 401/403 (was silently returning None); LocalVolts boundary translates LocalVoltsAPIError("auth failed") to ConfigEntryAuthFailed while preserving non-auth LocalVoltsAPIError semantics for other consumers; DWT-OE tags the failed provider before re-raising the existing PR-2 ConfigEntryAuthFailed.
• New _reauth_provider_id instance attribute on the coordinator — single source of truth for "which provider failed last." Set BEFORE raising; read by the ConfigFlow dispatcher via entry.runtime_data.coordinator (Phase 7 PR-1 typed runtime data path).
• config_flow.py: new async_step_reauth(entry_data) dispatcher routes to one of three per-provider sub-steps: async_step_reauth_amber, async_step_reauth_localvolts, async_step_reauth_dwt_oe. Unknown tag aborts cleanly with reason="reauth_provider_unknown".
• Each sub-step pre-fills existing fields, validates the new credential against a live probe (Amber /v1/sites, LocalVolts fetch_recent_intervals, OpenElectricity SDK fetch_current_price), and calls async_update_reload_and_abort on success. Entry id, options, and runtime_data accumulators (daily_cost_history, _saving_month_aud, _daily_wins) are preserved.
• API keys NEVER appear in log messages, UI errors, or exception strings — extends the existing PR-2 OpenElectricity scrubber pattern. Test test_amber_auth_failed_message_does_not_contain_key uses caplog to verify.
• strings.json + translations/en.json byte-identical mirror with three new reauth step descriptions, invalid_credentials error, and reauth_provider_unknown / reauth_successful abort reasons.

Stacked on PR #89 (Phase 7 PR-4). GitHub auto-retargets up the chain (PR #86 → #87 → #88 → #89 → this) as Phase 7 merges.

Test plan

• pytest --tb=short -q → 910 passed (884 baseline + 26 new)
• ruff check coordinator.py config_flow.py tests/test_reauth.py → clean
• diff strings.json translations/en.json → byte-identical
• Manual UAT via Playwright on homeassistant.local:8123:
  • Revoke active Amber API key → entry transitions to "Reauthorize" toast/repair → reauth flow accepts a new key
  • Confirm sensor.pricehawk_today_cost continues from yesterday's accumulated value (NOT reset to 0)
  • LocalVolts: rotate partner ID → multi-field reauth form pre-fills + accepts new combo
  • DWT-OE: invalid-key path surfaces invalid_api_key error (skip if no waitlisted OE key available)

Decisions: DECISIONS.md > D-P8-1.

Closes: Phase 8 / PR-5. Foundation for PR-6 reconfigure (consumes same dispatcher pattern).