feat(flights): cumulative fuel burned + CO2 emitted per flight

Pre-fix the emissions tooltip only showed the per-hour *rate* — what most
users actually want is the cumulative *amount* burned. This adds running
totals computed by multiplying the model-based rate by the elapsed
observation time since we first saw the airframe.

New module ``flight_observations.py``:
* Tracks first_seen_at + last_seen_at per icao24 hex.
* Re-opens a fresh session when an aircraft is unseen for > 15 min
  (treated as a new flight — landed and took off, or transited a dead
  zone). Prevents the cumulative counter from resetting mid-flight if
  the trail-rendering cache prunes the trail.
* Clamps elapsed time to 24h max so clock skew can't produce comically
  large numbers.
* Pruned every 5 min via a new scheduler job (mirrors ais_prune cadence).

flights.py + military.py emission enrichment now also attaches:
* observed_seconds — how long we've been tracking this airframe.
* fuel_gallons_burned — rate * elapsed_h.
* co2_kg_emitted — rate * elapsed_h.

The existing per-hour rate fields stay in the dict for backward compat
and are shown as small secondary context in the tooltip.

Frontend EmissionsEstimateBlock (NewsFeed.tsx) now prominently shows
the cumulative totals with the rate as smaller context underneath plus
"Observed in flight for Xh Ym". When observed_seconds is 0 (first refresh)
it renders "Just observed · totals will appear on next refresh" instead
of a misleading "0 gal".

12 backend tests cover record/accumulate/reset, the 24h clamp, prune,
case-insensitive key normalization, and end-to-end emission integration
in _classify_and_publish.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/services/fetchers/flights.py

@@ -17,6 +17,7 @@ from services.network_utils import fetch_with_curl
 from services.fetchers._store import latest_data, _data_lock, _mark_fresh
 from services.fetchers.plane_alert import enrich_with_plane_alert, enrich_with_tracked_names
 from services.fetchers.emissions import get_emissions_info
+from services.fetchers.flight_observations import record_observation as _record_flight_observation
 from services.fetchers.retry import with_retry
 from services.fetchers.route_database import lookup_route
 from services.fetchers.aircraft_database import lookup_aircraft_type
@@ -601,6 +602,22 @@ def _classify_and_publish(all_adsb_flights):
         if model:
             emi = get_emissions_info(model)
             if emi:
+                # Cumulative fuel/CO2: multiply the per-hour rate by how
+                # long we've been observing this airframe. Users want to
+                # see the *amount* burned, not just the rate. If we've
+                # never seen this hex before, observed_seconds is 0 and
+                # the cumulative values are 0 until the next refresh —
+                # the rate is still useful info on its own.
+                observed_seconds = _record_flight_observation(
+                    f.get("icao24") or ""
+                )
+                elapsed_h = observed_seconds / 3600.0
+                emi = {
+                    **emi,
+                    "observed_seconds": observed_seconds,
+                    "fuel_gallons_burned": round(emi["fuel_gph"] * elapsed_h, 1),
+                    "co2_kg_emitted": round(emi["co2_kg_per_hour"] * elapsed_h, 1),
+                }
                 f["emissions"] = emi
 
         callsign = f.get("callsign", "").strip().upper()