feat(mqtt): store F-070 crash detail and group crashes fleet-wide

Firmware F-070 adds a fuller `crash` object (backtrace, backtrace_corrupted,
elf_sha256) and a new `pre_crash` snapshot (uptime, heap stats, optional
abort_msg) to boot_report and to telemetry.get_boot_history entries.

- Migration c9d0e1f2a3b4: device_boot_events gains crash / pre_crash JSONB,
  stored verbatim so later additive fields need no migration. The legacy
  crash_* columns are still filled; old rows and old firmware are unchanged.
  JSON is bound as CAST(CAST(:x AS TEXT) AS JSONB): a bare JSONB cast makes
  asyncpg JSON-encode the already-encoded string a second time.
- boot_report ingestion passes both objects through.
- telemetry.get_boot_history replies (control/ack) are merged into boot
  history whoever sent the command: an entry matches an existing row on
  boot_count + reset_reason + time within 30 min (boot_count alone is not
  unique - the counter gets reset), and only fills crash/pre_crash the row
  lacks; unmatched entries are boots we never saw live and are inserted at
  the device's timestamp; entries without ts are skipped.
- GET /api/mqtt/crash-groups: fault boots grouped by abort_msg with hex
  addresses stripped, else task + exception cause, else reset reason. When
  abort_msg is present, pc/exc_cause describe abort() itself and are
  ignored for grouping.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/mqtt/models.py

@@ -129,6 +129,9 @@ class BootEventEntry(BaseModel):
     crash_pc: Optional[int] = None
     crash_exc_cause: Optional[int] = None
     crash_exc_vaddr: Optional[int] = None
+    # Firmware F-070 objects, verbatim — None for older firmware/rows.
+    crash: Optional[Dict[str, Any]] = None
+    pre_crash: Optional[Dict[str, Any]] = None
     occurred_at: str
 
 
@@ -137,6 +140,29 @@ class BootEventListResponse(BaseModel):
     total: int
 
 
+class CrashGroupDevice(BaseModel):
+    device_serial: str
+    count: int
+    last_at: str
+
+
+class CrashGroup(BaseModel):
+    key: str
+    kind: str             # "abort" | "exception" | "reason"
+    title: str
+    count: int
+    device_count: int
+    first_at: str
+    last_at: str
+    latest: BootEventEntry
+    devices: List[CrashGroupDevice]
+
+
+class CrashGroupListResponse(BaseModel):
+    groups: List[CrashGroup]
+    total_crashes: int
+
+
 class PingSampleEntry(BaseModel):
     id: int
     device_serial: str