feat: bundled MC2MQTT broker presets (waev, letsmesh) + format family

Introduces a 'set format and forget' workflow for MQTT brokers. Users
reference a bundled preset by name inside the existing brokers: list,
and the package supplies the endpoints, audiences, and TLS settings.
Endpoint changes ship via 'pip install -U' instead of manual edits.

What changes
- New repeater/presets/ package with a tiny lazy YAML loader and two
  bundled presets: waev (mqtt-{a,b}.waev.app) and letsmesh (EU + US).
- New format-family constant MC2MQTT_FORMATS = ('meshcoretomqtt',
  'letsmesh', 'waev') replaces the inline tuple in topic resolution.
  The legacy 'mqtt' format keeps its custom-topic semantics unchanged.
- Two-pass broker assembly in mqtt_handler.py: pass 1 expands every
  {preset: <name>} entry inline; pass 2 collapses duplicates by name
  with later-wins semantics. Place override entries AFTER preset
  entries.
- Hard-coded LETSMESH_BROKERS constant deleted; its data now lives in
  repeater/presets/letsmesh.yaml.
- convert_letsmesh_to_broker_config() collapsed from ~70 to ~25 lines
  by emitting {preset: letsmesh} plus disable overrides for unwanted
  brokers. Honors broker_index in (-1, 0, 1), additional_brokers, and
  enabled flag exactly as before.
- update_mqtt_config API endpoint accepts {preset: <name>} entries and
  passes them through unchanged so the web UI can author them when the
  frontend is updated.
- config.yaml.example documents the preset entry shape, the override
  rule, and the format family hierarchy.
- pyproject.toml ships presets/*.yaml as package data.

How to use
  mqtt_brokers:
    iata_code: "LAX"
    brokers:
      - preset: waev

  # Override a single preset broker:
  brokers:
    - preset: waev
    - name: waev-b
      enabled: false

Tests
- tests/test_presets.py: 9 tests covering loader, expand/merge,
  MC2MQTT topic-family parity, and parametrized legacy migration.

Co-Authored-By: Oz <oz-agent@warp.dev>

repeater/web/api_endpoints.py

@@ -1224,6 +1224,13 @@ class APIEndpoints:
                 for i, b in enumerate(brokers):
                     if not isinstance(b, dict):
                         return self._error(f"Broker at index {i} must be an object")
+
+                    # Bundled preset reference: {preset: <name>}. Pass through
+                    # unchanged - the MQTT handler expands it on next start.
+                    if "preset" in b and "name" not in b:
+                        validated.append({"preset": str(b["preset"]).strip()})
+                        continue
+
                     for field in ("name", "host", "port", "format"):
                         if not b.get(field, ""):
                             return self._error(f"Broker at index {i} missing required field: {field}")