feat: add PDF reports, timezone management, GPS QR codes, and fix GPS tracking gaps

Issue #1: QR button on GPS Devices tab for re-enrollment
Issue #2: App-wide timezone setting with TimezoneContext, useFormattedDate hook,
  and admin timezone selector. All date displays now respect the configured timezone.
Issue #3: PDF export for Accountability Roster using @react-pdf/renderer with
  professional styling matching VIPSchedulePDF. Added Signal send button.
Issue #4: Fixed GPS "teleporting" gaps - syncPositions now fetches position history
  per device instead of only latest position. Changed cron to every 30s, added
  unique constraint on deviceId+timestamp for deduplication, lowered min interval to 10s.

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

backend/prisma/migrations/20260208000000_add_gps_location_unique_constraint/migration.sql

@@ -0,0 +1,12 @@
+-- Delete duplicate rows keeping the first entry (by id) for each deviceId+timestamp pair
+DELETE FROM "gps_location_history" a
+USING "gps_location_history" b
+WHERE a."id" > b."id"
+  AND a."deviceId" = b."deviceId"
+  AND a."timestamp" = b."timestamp";
+
+-- Drop the existing index that covered deviceId+timestamp (non-unique)
+DROP INDEX IF EXISTS "gps_location_history_deviceId_timestamp_idx";
+
+-- CreateIndex (unique constraint replaces the old non-unique index)
+CREATE UNIQUE INDEX "gps_location_history_deviceId_timestamp_key" ON "gps_location_history"("deviceId", "timestamp");

backend/prisma/schema.prisma

@@ -430,7 +430,7 @@ model GpsLocationHistory {
   createdAt       DateTime  @default(now())
 
   @@map("gps_location_history")
-  @@index([deviceId, timestamp])
+  @@unique([deviceId, timestamp]) // Prevent duplicate position records
   @@index([timestamp]) // For cleanup job
 }