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/f6a7b8c9d0e1_device_health_tab.py

@@ -0,0 +1,82 @@
+"""device health tab — boot events, heartbeat free_heap, ping samples
+
+Adds three pieces of schema needed for the device Health tab:
+
+1. device_boot_events — structured, insert-only history of the firmware's
+   boot_report MQTT event (vesper/{uid}/status/info, type="boot_report").
+   Previously this payload was flattened into a single device_logs text line
+   with all structured fields (boot_count, crash detail) discarded on arrival.
+   This table gives the console a real timeline to query and chart against,
+   instead of parsing log strings.
+
+2. heartbeats.free_heap — firmware now includes free_heap on every 30s
+   heartbeat (previously only available once per boot via boot_report), so
+   the console can chart heap trend instead of seeing one point per boot.
+
+3. device_ping_samples — backend-computed RTT samples. The firmware's ping
+   command now echoes back a caller-supplied timestamp; the backend pings
+   each online device on an interval and records (now - echoed_ts) here.
+   A dedicated table rather than reusing `commands` because `commands` isn't
+   shaped for time-series charting (mixed command types, no fast per-device
+   time-range query path) and pruning ping history independently of other
+   command history is desirable.
+
+Revision ID: f6a7b8c9d0e1
+Revises: e5f6a7b8c9d0
+Create Date: 2026-07-16 00:00:00.000000
+"""
+from typing import Sequence, Union
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "f6a7b8c9d0e1"
+down_revision: Union[str, None] = "e5f6a7b8c9d0"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "device_boot_events",
+        sa.Column("id",            sa.BigInteger(), primary_key=True, autoincrement=True),
+        sa.Column("device_serial", sa.String(128), nullable=False),
+        sa.Column("boot_count",    sa.Integer(),   nullable=True),
+        sa.Column("reset_reason",  sa.String(64),  nullable=True),
+        sa.Column("is_fault",      sa.Boolean(),   nullable=False, server_default=sa.false()),
+        sa.Column("free_heap",     sa.Integer(),   nullable=True),
+        sa.Column("crash_task",      sa.String(64),   nullable=True),
+        sa.Column("crash_pc",        sa.BigInteger(), nullable=True),
+        sa.Column("crash_exc_cause", sa.Integer(),    nullable=True),
+        sa.Column("crash_exc_vaddr", sa.BigInteger(), nullable=True),
+        sa.Column("occurred_at",   sa.DateTime(timezone=True), nullable=False,
+                  server_default=sa.func.now()),
+    )
+    op.create_index(
+        "idx_device_boot_events_serial_occurred",
+        "device_boot_events",
+        ["device_serial", sa.text("occurred_at DESC")],
+    )
+
+    op.add_column("heartbeats", sa.Column("free_heap", sa.Integer(), nullable=True))
+
+    op.create_table(
+        "device_ping_samples",
+        sa.Column("id",            sa.BigInteger(), primary_key=True, autoincrement=True),
+        sa.Column("device_serial", sa.String(128), nullable=False),
+        sa.Column("rtt_ms",        sa.Integer(),   nullable=False),
+        sa.Column("sampled_at",    sa.DateTime(timezone=True), nullable=False,
+                  server_default=sa.func.now()),
+    )
+    op.create_index(
+        "idx_device_ping_samples_serial_sampled",
+        "device_ping_samples",
+        ["device_serial", sa.text("sampled_at DESC")],
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("idx_device_ping_samples_serial_sampled", table_name="device_ping_samples")
+    op.drop_table("device_ping_samples")
+    op.drop_column("heartbeats", "free_heap")
+    op.drop_index("idx_device_boot_events_serial_occurred", table_name="device_boot_events")
+    op.drop_table("device_boot_events")