Add read-only OpenEMS data-source plugin

## Problem

MPM currently has no supported way to import customer energy usage from an OpenEMS-managed mini-grid. Operators need to map OpenEMS meter components to existing MPM meters and make interval consumption available in MPM without affecting billing or meter control.

## Proposed solution

Add a read-only OpenEMS plugin that:

- connects tenant configurations to one or more OpenEMS Backends through the Backend-to-Backend JSON-RPC API;
- maps multiple OpenEMS Edges to an MPM mini-grid;
- provides a customer-first workflow in which an operator selects a customer
and then explicitly selects one of that customer's existing MPM meters;
- persists the meter mapping as the reading identity and derives the customer
through MPM's existing device ownership relationship;
- imports 15-minute consumption into the existing meter-reading system;
- uses historical energy queries when supported and cumulative-counter polling otherwise;
- handles retries, duplicate delivery, counter resets, gaps, and partial failures;
- applies OpenEMS-only tiered retention;
- provides authenticated connection, mapping, synchronization, CSV import, health, and usage workflows;
- adds an operator dashboard filtered by mini-grid and date.

## Scope

V1 is informational only. Imported readings must not alter customer balances, tariffs, invoices, tokens, device state, or OpenEMS configuration. Live power, voltage, and remote connect/disconnect are excluded.

## Acceptance criteria

- Readings appear in MPM within 20 minutes of their interval.
- Duplicate or retried delivery creates no duplicate readings.
- Tenant data remains isolated.
- Disabling the plugin prevents new synchronization jobs.
- A target-instance pilot reconciles daily energy totals within 0.1% or 0.01 kWh, whichever is larger.
- The design supports up to 5,000 mapped meters per tenant.

## Rollout requirements

Before production rollout, capture anonymized fixtures from the target OpenEMS version, run the 5,000-meter scale check, and complete a seven-day mini-grid reconciliation pilot.

Start by reviewing MPM’s existing meter-reading system and device ownership relationship, then the OpenEMS Backend-to-Backend JSON-RPC API. Define the plugin’s integration, persistence, synchronization, dashboard, and CSV workflows before implementation; done means the stated isolation, reliability, reconciliation, retention, and scale acceptance criteria are met.

Written by the indexing model from the issue text.