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/d4e5f6a7b8c9_device_alert_events.py

@@ -0,0 +1,48 @@
+"""device_alert_events
+
+Adds an insert-only history log of device alert transitions (WARNING/CRITICAL/
+FAILED only — CLEARED is not logged here). device_alerts remains the current-
+state table and is untouched; this is purely additive, used to answer "when
+was the most recent issue on this device" even after it has been resolved.
+
+Also adds an optional rssi column to heartbeats, for a signal-strength
+indicator once firmware starts reporting it on the heartbeat payload.
+
+Revision ID: d4e5f6a7b8c9
+Revises: c3d4e5f6a7b8
+Create Date: 2026-07-13 00:00:00.000000
+"""
+from typing import Sequence, Union
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "d4e5f6a7b8c9"
+down_revision: Union[str, None] = "c3d4e5f6a7b8"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "device_alert_events",
+        sa.Column("id",            sa.BigInteger(), primary_key=True, autoincrement=True),
+        sa.Column("device_serial", sa.String(128), nullable=False),
+        sa.Column("subsystem",     sa.String(128), nullable=False),
+        sa.Column("state",         sa.String(64),  nullable=False),
+        sa.Column("message",       sa.Text(),      nullable=True),
+        sa.Column("occurred_at",   sa.DateTime(timezone=True), nullable=False,
+                  server_default=sa.func.now()),
+    )
+    op.create_index(
+        "idx_device_alert_events_serial_occurred",
+        "device_alert_events",
+        ["device_serial", sa.text("occurred_at DESC")],
+    )
+
+    op.add_column("heartbeats", sa.Column("rssi", sa.Integer(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("heartbeats", "rssi")
+    op.drop_index("idx_device_alert_events_serial_occurred", table_name="device_alert_events")
+    op.drop_table("device_alert_events")