Add OCPP charger support (closes #44)

[kljakobsen]

Implements a charger backend for OCPP-managed charge points via the lbbrhzn/ocpp integration. Tested end-to-end on a Garo Entity Pro using a Shelly 3EM as the meter source (as custom meter).

Design notes for anyone maintaining this:

• OCPP unique-ids are dot-separated (e.g. number.ocpp..maximum_current), so the base HaDevice._get_entity_id_by_key helper's underscore-suffix match never works. OcppCharger ships its own dot-aware _get_ocpp_entity_id that matches by HA domain and by checking the key against the dot-separated unique-id parts.

• IEC 61851 forbids charging below 6A. The upstream allocator can emit any value 0..max during overcurrent events. set_current_limit snaps any request in (0, 6) to 0A, which OCPP interprets as "pause", and the charger transitions to SuspendedEVSE.

• The pause must not look like a manual override. The upstream power_allocator.detect_manual_override() compares the slider (0A after the snap) against last_applied_current (the pre-snap value like 4A), would see a mismatch and adopt 0A as the new ceiling -- permanently locking the charger off. OcppCharger tracks _last_requested and, while the charger is in SuspendedEVSE, get_current_limit reports that pre-snap value back so the override detector sees no change.

• Profile replacement alone does not resume on most chargers. Once SuspendedEVSE, the contactor is de-energised and the firmware waits for a fresh RemoteStartTransaction. When set_current_limit is called with >=6A while paused, we push the new profile first, then toggle the charge_control switch off -> on to issue RemoteStop + RemoteStart and force a new transaction under the just-updated profile.

• OCPP exposes one charge-point-wide current setting, so has_synced_phase_limits() is always True and set_current_limit uses the lowest of the per-phase values.

• set_phase_mode is a validated no-op (OCPP 1.6 has no standardised single/three-phase toggle).

Validation:

• All 33 new OcppCharger tests pass.
• All 138 charger tests pass.
• ruff check . clean; ruff format --check . clean.
• Verified end-to-end on real hardware (Garo Entity Pro via lbbrhzn/ocpp and Shelly 3EM): multiple pause/resume cycles in a single session, no "Manual override detected" log lines during pauses, contactor reliably re-energised on resume.

Files changed:
custom_components/evse_load_balancer/chargers/ocpp_charger.py (new)
custom_components/evse_load_balancer/chargers/init.py (register)
custom_components/evse_load_balancer/config_flow.py (picker)
custom_components/evse_load_balancer/const.py (domain)
tests/chargers/test_ocpp_charger.py (new)
README.md (table)