Add TFR warning banner for airspace restrictions

Fetches active Temporary Flight Restrictions from the FAA website,
filters by configured state (LOCATION_STATE env var), and displays
a red warning banner at the top of the dashboard when TFRs are present.
Data is cached for 30 minutes and degrades gracefully if the FAA is unreachable.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/model/tfr.go

@@ -0,0 +1,12 @@
+package model
+
+// TFR represents a Temporary Flight Restriction
+type TFR struct {
+	Date        string `json:"date"`
+	NotamID     string `json:"notam_id"`
+	Facility    string `json:"facility"`
+	State       string `json:"state"`
+	Type        string `json:"type"`
+	Description string `json:"description"`
+	DetailURL   string `json:"detail_url"`
+}