feat: add single-profile settings table and store layer

Add Profile table to store Garmin credentials and engine configuration
parameters. Implements GetProfile() and UpdateProfile() store methods with
comprehensive test coverage. The profile singleton row is automatically
initialized on migration and persists all configuration state.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/internal/store/profile_test.go

@@ -0,0 +1,48 @@
+package store
+
+import (
+	"context"
+	"testing"
+)
+
+func TestProfile_DefaultsThenUpdate(t *testing.T) {
+	db := openTestDB(t)
+	ctx := context.Background()
+
+	p, err := db.GetProfile(ctx)
+	if err != nil {
+		t.Fatalf("GetProfile: %v", err)
+	}
+	if p.RollingWindowDays != 90 {
+		t.Errorf("RollingWindowDays = %d, want 90 (migration default)", p.RollingWindowDays)
+	}
+	if p.HRZone1MinPct != 50 || p.HRZone5MaxPct != 100 {
+		t.Errorf("zone defaults = %+v, want Z1 min=50, Z5 max=100", p)
+	}
+
+	maxHR, restingHR := 190.0, 50.0
+	p.GarminEmail = "runner@example.com"
+	p.GarminPassword = "hunter2"
+	p.RollingWindowDays = 120
+	p.MaxHeartRate = &maxHR
+	p.RestingHeartRate = &restingHR
+	p.IntervalWarmupMinutes = 8
+
+	if err := db.UpdateProfile(ctx, p); err != nil {
+		t.Fatalf("UpdateProfile: %v", err)
+	}
+
+	got, err := db.GetProfile(ctx)
+	if err != nil {
+		t.Fatalf("GetProfile after update: %v", err)
+	}
+	if got.GarminEmail != "runner@example.com" || got.RollingWindowDays != 120 {
+		t.Errorf("got = %+v, want updated email/window", got)
+	}
+	if got.MaxHeartRate == nil || *got.MaxHeartRate != 190 {
+		t.Errorf("MaxHeartRate = %v, want 190", got.MaxHeartRate)
+	}
+	if got.IntervalWarmupMinutes != 8 {
+		t.Errorf("IntervalWarmupMinutes = %v, want 8", got.IntervalWarmupMinutes)
+	}
+}