v1.12.0 - Token caching

What changed

The integration now caches the AEL session token across coordinator polls instead of re-running the full 5-step authentication every hour. Steady-state load drops from 8 requests/poll (5 auth + 3 fetch) to 3 requests/poll (just the data fetches). The full auth handshake is only re-run when the cached token actually stops working.

Why it matters

• Less load on the SOMEI/Veolia portal — friendlier behaviour shared with all integration users
• Faster polls (no need to walk through 5 sequential HTTP round-trips first)
• Same reliability: a 401/403 from any fetch transparently triggers re-authentication and one retry

Behavioural details

• authenticate() is now idempotent — short-circuits when an AEL token is already cached. The coordinator's existing call pattern (authenticate(); fetch()) becomes free after the first cycle.
• 401/403 responses on fetches now raise the new EauxDeMarseilleSessionExpiredError (subclass of EauxDeMarseilleApiError, so existing catch clauses still work).
• The high-level client funnels both fetch() and fetch_monthly_range() through a small recovery wrapper:
  • Authenticates first if no token is cached.
  • On EauxDeMarseilleSessionExpiredError, invalidates the cache, re-authenticates, retries the action exactly once.
  • Any other exception (auth failure, transport, generic 4xx, 5xx after retries) propagates unchanged.
• Bounded recovery: a second 401 surfaces normally to the coordinator → ConfigEntryAuthFailed → reauth flow. No infinite loop.

Verified

• Full local validation (ruff, mypy strict, pytest 36 passed — 6 new tests for session recovery)
• All 5 CI jobs green
• Live smoke test on a Raspberry Pi against SEM (Ventabren contract)

Test coverage added

• test_authenticate_is_idempotent
• test_fetch_skips_redundant_auth
• test_fetch_recovers_from_401
• test_403_also_triggers_recovery
• test_repeated_session_expiry_propagates
• test_other_4xx_does_not_trigger_reauth — confirms a 404 does NOT trigger re-auth (would mask real bugs)

Closes #12.