feat(mqtt): add device health telemetry and migrate to v2 topic spec

Two efforts that landed together because the v2 topic work extends
tables the health-telemetry effort added days earlier in the same
files/functions, making them impractical to separate cleanly:

Health/diagnostics telemetry (schema, Jul 13-17):
- New Postgres tables: device_alert_events, device_boot_events,
  device_ping_samples, device_diagnostics_reports, plus a `source`
  column on device_logs to distinguish log origins
- Query/service layer in pg_mqtt.py and database/__init__.py for
  inserting and listing this history, plus a "latest metrics" endpoint
  combining most-recent diagnostics + ping RTT per device
- mqtt/router.py gains list endpoints for alert/boot/ping/diagnostics
  history, consumed by the upcoming Health tab

MQTT v2 topic migration (Sep 21):
- Heartbeat payload flattened per vesper_mqtt_topic_spec_v2.md, adding
  rssi/free_heap/state/ok fields
- Command replies move to control/ack, device-initiated events to
  control/reports; mqtt/client.py subscribes to the new topic set and
  runs a ping_loop (wired up in main.py) for RTT sampling
- mqtt/logger.py and pg_mqtt.py updated to parse and persist the new
  payload shape alongside the legacy fields for backwards compatibility

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/alembic/versions/e5f6a7b8c9d0_device_logs_source.py

@@ -0,0 +1,39 @@
+"""device_logs source column
+
+Adds a 'source' column to device_logs distinguishing the debug log stream
+(vesper/{id}/logs, source='log') from the general info stream
+(vesper/{id}/status/info, source='info'), which was previously discarded
+after only being written to the server debug log. Existing rows default to
+'log' since that's the only source ever persisted before this migration.
+
+Adding a column to a partitioned parent table applies it to all existing
+and future partitions automatically.
+
+Revision ID: e5f6a7b8c9d0
+Revises: d4e5f6a7b8c9
+Create Date: 2026-07-14 00:00:00.000000
+"""
+from typing import Sequence, Union
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "e5f6a7b8c9d0"
+down_revision: Union[str, None] = "d4e5f6a7b8c9"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.execute("""
+        ALTER TABLE device_logs
+        ADD COLUMN source TEXT NOT NULL DEFAULT 'log'
+    """)
+    op.execute("""
+        CREATE INDEX idx_device_logs_source
+        ON device_logs(device_serial, source, received_at DESC)
+    """)
+
+
+def downgrade() -> None:
+    op.execute("DROP INDEX IF EXISTS idx_device_logs_source")
+    op.execute("ALTER TABLE device_logs DROP COLUMN source")