Add device-certificate mTLS auth, live position tracking, and API docs

Introduces a CA/PKI module so field devices can authenticate to Mosquitto
over TLS (8883) with per-device client certificates (CN = serial number)
instead of a shared password, with matching Devices/MQTT-Certs UI. Adds
live transmitter position tracking alongside logged points, an MQTTS
transport option in the simulator for exercising the real cert-auth path,
and Swagger API docs at /api/docs.

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

backend/prisma/schema.prisma

@@ -142,15 +142,45 @@ model Device {
   // fetches this via GET /api/devices/:serial/status to show on its own screen.
   disabledReason String?
   lastSeenAt   DateTime? @db.Timestamptz(6)
+  // Most recent known position, from either a "status" ping (live-only, never
+  // persisted as a LocatePoint) or a "log" point — whichever is newest. Lets
+  // the devices page show current position without waiting for a log write.
+  lastPosition   Json?
+  lastPositionAt DateTime? @db.Timestamptz(6)
   createdAt    DateTime  @default(now()) @db.Timestamptz(6)
   updatedAt    DateTime  @updatedAt @db.Timestamptz(6)
 
-  org    Organization  @relation(fields: [orgId], references: [id], onDelete: Cascade)
-  points LocatePoint[]
+  org         Organization       @relation(fields: [orgId], references: [id], onDelete: Cascade)
+  points      LocatePoint[]
+  certificate DeviceCertificate?
 
   @@map("devices")
 }
 
+// A client certificate issued to a device for mTLS auth on the broker's 8883
+// listener; the cert's CN (= serialNumber) becomes the MQTT username. The CA
+// key/server key never touch the DB (see PkiService) — only device
+// certs/keys are stored here, mirroring how the reference implementation
+// (MQTT_DEVICE_AUTH.md) does it. One active cert per device; issuing a new
+// one requires deleting this row first. No revokedAt: deleting the row is
+// the only "revoke" action there is, and it isn't broker-enforced either way
+// (no CRL/OCSP), so a soft-delete flag would misleadingly imply otherwise.
+model DeviceCertificate {
+  id             String   @id @default(cuid())
+  deviceId       String   @unique
+  serialNumber   String
+  commonName     String
+  certificatePem String   @db.Text
+  privateKeyPem  String   @db.Text
+  fingerprint    String
+  issuedAt       DateTime @default(now()) @db.Timestamptz(6)
+  expiresAt      DateTime @db.Timestamptz(6)
+
+  device Device @relation(fields: [deviceId], references: [id], onDelete: Cascade)
+
+  @@map("device_certificates")
+}
+
 model LocatePoint {
   id          BigInt      @id @default(autoincrement())
   jobId       String