Add SIM868 cellular fallback for tracking uploads (untested, no SIM card)

modules/app_cellular.py: uses the SIM868's built-in AT-command HTTP(S)
client (AT+HTTPINIT/HTTPPARA/HTTPACTION/HTTPREAD) instead of a real
ppp0 interface, since the module's GNSS (polled continuously by
app_gps.py) and its GSM/GPRS radio share the same physical UART
(/dev/serial0) - a pppd/chat dial-up would take over that port
entirely and starve GNSS polling. Staying in AT-command mode lets
cellular HTTP calls interleave safely through the same serial lock.

Only carries small JSON tracking batches, never dashcam video -
cellular is far too slow/limited for that, and the modem's HTTP AT
interface has too small an internal buffer anyway. Dashcam uploads
remain WiFi/Ethernet-only, unchanged.

modules/app_upload.py: tracking-batch upload always tries WiFi/
Ethernet first; only on a connection-level failure (not an HTTP error
status, which means the server was reached) does it retry over
cellular. Exposes last_network_path/cellular_fallback_configured in
/api/upload/status; UI shows which path the last batch used.

Feature-flagged off by default via new YARIS_CELLULAR_ENABLED/
YARIS_CELLULAR_APN/_APN_USER/_APN_PASS keys in db/upload_creds.env
(all blank by default) - has zero effect on the existing WiFi upload
path unless explicitly configured. Regression-tested against the real
deployed server that normal WiFi uploads are unaffected while
cellular stays unconfigured.

*** UNTESTED against real cellular hardware - no SIM card was
*** available. See docstrings in app_cellular.py and PLAN.md for
*** exactly what needs validation once a SIM card is available.

Co-authored-by: Copilot <[email protected]>

templates/dash.html

@@ -91,7 +91,9 @@
                             <p class="text-muted small mt-2 mb-0">
                                 Trip/GPS log uploads always stay on. Turning this off only pauses
                                 the (larger) dashcam video uploads - useful on a slow or metered
-                                connection.
+                                connection. If configured, small trip-log batches can also fall
+                                back to the SIM868's cellular data when WiFi/Ethernet is
+                                unreachable - dashcam video never uses cellular.
                             </p>
                         </div>
                     </div>