feat(user): add async GDPR export endpoint with 12h cache and 25 tests

Add GET /api/user/gdpr-export handler (GetGDPRExportHandler) with async background generation — navigation away doesn't cancel. 12h in-memory cache with 5-minute cleanup ticker. Three cache states: MISS (triggers generation), HIT (returns cached data), GENERATING (concurrent request while building). Route registered without RequireVerified middleware so unverified users can export. 25 tests covering: auth rejection, cache miss/hit/generating/expired states, anonymize_user child table scrubbing (social logins, saved cards, verification codes, time blockers, edit notes, notification prefs), export_all_user_data 16-section export, AnonymizeStaleGuestAccounts field scrubbing.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/user/gdpr_export.go

@@ -0,0 +1,92 @@
+package user
+
+import (
+	"context"
+	"encoding/json"
+	"log"
+	"net/http"
+	"sync"
+	"time"
+
+	"crussell/db"
+	"crussell/mw"
+)
+
+var (
+	gdprExportCache   = make(map[string]*gdprCacheEntry)
+	gdprExportCacheMu sync.RWMutex
+)
+
+type gdprCacheEntry struct {
+	data       json.RawMessage
+	expiresAt  time.Time
+	generating bool
+}
+
+func init() {
+	go func() {
+		ticker := time.NewTicker(5 * time.Minute)
+		defer ticker.Stop()
+		for range ticker.C {
+			gdprExportCacheMu.Lock()
+			now := time.Now()
+			for k, v := range gdprExportCache {
+				if now.After(v.expiresAt) {
+					delete(gdprExportCache, k)
+				}
+			}
+			gdprExportCacheMu.Unlock()
+		}
+	}()
+}
+
+// GET /api/user/gdpr-export
+func GetGDPRExportHandler(w http.ResponseWriter, r *http.Request) {
+	userID, ok := mw.GetUserID(r.Context())
+	if !ok {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	gdprExportCacheMu.Lock()
+	if entry, found := gdprExportCache[userID]; found && time.Now().Before(entry.expiresAt) {
+		if entry.generating {
+			gdprExportCacheMu.Unlock()
+			w.Header().Set("Content-Type", "application/json")
+			w.Header().Set("X-Cache", "GENERATING")
+			w.Write([]byte(`{"status":"generating"}`))
+			return
+		}
+		gdprExportCacheMu.Unlock()
+		w.Header().Set("Content-Type", "application/json")
+		w.Header().Set("X-Cache", "HIT")
+		w.Write(entry.data)
+		return
+	}
+
+	gdprExportCache[userID] = &gdprCacheEntry{generating: true}
+	gdprExportCacheMu.Unlock()
+
+	go func() {
+		var result json.RawMessage
+		err := db.DB.QueryRow(context.Background(), `SELECT export_all_user_data($1)`, userID).Scan(&result)
+		if err != nil {
+			log.Printf("GDPR export failed for user %s: %v", userID, err)
+			gdprExportCacheMu.Lock()
+			delete(gdprExportCache, userID)
+			gdprExportCacheMu.Unlock()
+			return
+		}
+
+		gdprExportCacheMu.Lock()
+		gdprExportCache[userID] = &gdprCacheEntry{
+			data:      result,
+			expiresAt: time.Now().Add(12 * time.Hour),
+		}
+		gdprExportCacheMu.Unlock()
+	}()
+
+	w.Header().Set("Content-Type", "application/json")
+	w.Header().Set("X-Cache", "MISS")
+	w.Write([]byte(`{"status":"generating"}`))
+}