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

[dmduran12]

What this does

Adds bundled MeshCoreToMQTT (MC2MQTT) broker presets so operators can subscribe
to a known mesh network with one line of config instead of copy-pasting
multi-line broker definitions. Endpoints ship with the package, so URL/audience
updates arrive via pip install -U.

Why

Today, anyone publishing to LetsMesh, Waev, or any future MC2MQTT-compatible
platform has to copy/paste the full broker definition into config.yaml. When
the operator changes endpoints upstream, every node operator has to edit
manually. Hard-coded LETSMESH_BROKERS constant in mqtt_handler.py was the
only "registry" — Python code, not data.

How to use

Minimal — set and forget

mqtt_brokers:
  iata_code: "LAX"
  brokers:
    - preset: waev

Multiple consumers

mqtt_brokers:
  brokers:
  iata_code: "LAX"
    - preset: waev
    - preset: letsmesh

Override a preset broker: Place override entries after the preset entry. Later wins on name collision.

mqtt_brokers:
  brokers:
    - preset: waev
    - name: waev-b
      enabled: false

Mix presets with a custom broker:

mqtt_brokers:
  brokers:
    - preset: waev
    - name: my-lan-mqtt
      enabled: true
      host: mqtt.lan
      port: 1883
      transport: tcp
      format: mqtt
      username: repeater
      password: secret

API Changes
POST /api/update_mqtt_config now accepts preset references inside brokers
A {preset: } entry is passed through unchanged; the MQTT handler
expands it on next start. All other broker entries are validated as before
(name/host/port/format required).

{
  "brokers": [
    {"preset": "waev"},
    {"name": "waev-b", "enabled": false}
  ]
}

Behavior contract

• Empty brokers: → empty (current behavior).
• Only ordinary entries → unchanged from today.
• [{preset: waev}] → both Waev brokers enabled.
• [{preset: waev}, {name: waev-b, enabled: false}] → both, waev-b disabled.
• [{preset: bogus}] → empty + warning log; daemon does not crash.
• Legacy letsmesh: block → produces the same broker set as before for every
prior broker_index value (-1 / 0 / 1).

What's deleted

• Hard-coded LETSMESH_BROKERS constant (~24 lines) — its data is now
repeater/presets/letsmesh.yaml.
• convert_letsmesh_to_broker_config() collapsed from ~70 lines to ~25 lines.
The legacy migrator now emits {preset: letsmesh} plus disable overrides
and lets the same expansion path handle the rest.

Net code change is negative outside the new loader.

Tests

tests/test_presets.py — 9 tests covering:

• Preset loader (list_presets, get_preset, unknown returns {}).
• Pass 1 expansion (inlines bundled brokers, drops unknown with warning).
• Pass 2 merge (override AFTER preset wins).
• Documented order rule (override BEFORE preset is silently overwritten — locks
the published rule so a future refactor can't quietly flip it).
• MC2MQTT topic-family parity vs legacy mqtt topic.
• Parametrized legacy broker_index in (-1, 0, 1) migration.

All 9 pass. Pre-existing test failures on dev are unrelated to this PR
(verified by re-running them on dev before and after).

note:
The frontend dropdown for the new meshcoretomqtt format value lands in a
follow-up frontend PR — that bundle lives in a separate repo. The backend
already accepts both shapes, so YAML-edit operators get the full feature today.

[rightup]

@Rigear this touches your work do you mind taking a look. thanks

[Rigear]

@rightup Changes look good to me.

Nice work @dmduran12