Add configurable pace-artifact filtering to replace the hardcoded cutoff

Two new Profile settings -- "minimum representative pace" and "minimum
representative time" -- replace the previous hardcoded 20:00/km cutoff. A
stretch of consecutive samples slower than the configured pace is now
dropped from the chart (and its Y-axis scale) only if it lasts no longer
than the configured time; a longer stretch is kept as a real stop or walk
break rather than noise. Defaults to 12:00/km and 3 seconds.

Caught a boundary bug while verifying against real data: a run lasting
exactly the threshold duration survived filtering because the comparison
used strict "<" instead of "<=", contradicting "not lasting more than N
seconds" (which should include exactly N).

backend/internal/store/migrations/0010_pace_artifact_filter.sql

@@ -0,0 +1,9 @@
+-- Filters brief pace "artifacts" (e.g. GPS/motion still settling right as
+-- recording starts, before the run itself begins) out of the Review
+-- Queue's pace chart: a stretch of samples slower than
+-- min_representative_pace_sec_per_km is dropped unless it persists for at
+-- least min_representative_time_seconds, in which case it's treated as a
+-- real stop or walk break, not noise. Defaults match the values used to
+-- design this feature (12:00/km, 3 seconds).
+ALTER TABLE profile ADD COLUMN min_representative_pace_sec_per_km REAL NOT NULL DEFAULT 720;
+ALTER TABLE profile ADD COLUMN min_representative_time_seconds REAL NOT NULL DEFAULT 3;

backend/internal/store/profile.go

@@ -32,6 +32,15 @@ type Profile struct {
 	// don't use these.
 	WarmupMinutes, CooldownMinutes float64
 
+	// MinRepresentativePaceSecPerKm/MinRepresentativeTimeSeconds drive the
+	// Review Queue pace chart's artifact filter: a stretch of samples slower
+	// than MinRepresentativePaceSecPerKm is dropped from the chart (and its
+	// Y-axis scale) unless it persists for at least
+	// MinRepresentativeTimeSeconds, in which case it's treated as a real
+	// stop or walk break rather than noise (e.g. GPS/motion still settling
+	// right as recording starts, before the run itself begins).
+	MinRepresentativePaceSecPerKm, MinRepresentativeTimeSeconds float64
+
 	CreatedAt, UpdatedAt string
 }
 
@@ -41,6 +50,7 @@ const profileColumns = `
 	hr_zone3_min_pct, hr_zone3_max_pct, hr_zone4_min_pct, hr_zone4_max_pct,
 	hr_zone5_min_pct, hr_zone5_max_pct,
 	warmup_minutes, cooldown_minutes,
+	min_representative_pace_sec_per_km, min_representative_time_seconds,
 	created_at, updated_at
 `
 
@@ -53,6 +63,7 @@ func (db *DB) GetProfile(ctx context.Context) (Profile, error) {
 		&p.HRZone3MinPct, &p.HRZone3MaxPct, &p.HRZone4MinPct, &p.HRZone4MaxPct,
 		&p.HRZone5MinPct, &p.HRZone5MaxPct,
 		&p.WarmupMinutes, &p.CooldownMinutes,
+		&p.MinRepresentativePaceSecPerKm, &p.MinRepresentativeTimeSeconds,
 		&p.CreatedAt, &p.UpdatedAt,
 	)
 	if err != nil {
@@ -72,6 +83,7 @@ func (db *DB) UpdateProfile(ctx context.Context, p Profile) error {
 			hr_zone3_min_pct=?, hr_zone3_max_pct=?, hr_zone4_min_pct=?, hr_zone4_max_pct=?,
 			hr_zone5_min_pct=?, hr_zone5_max_pct=?,
 			warmup_minutes=?, cooldown_minutes=?,
+			min_representative_pace_sec_per_km=?, min_representative_time_seconds=?,
 			updated_at=datetime('now')
 		WHERE id = 1`,
 		p.GarminEmail, p.GarminPassword, p.RollingWindowDays, p.BackfillHorizonDays, p.MaxHeartRate, p.RestingHeartRate,
@@ -79,6 +91,7 @@ func (db *DB) UpdateProfile(ctx context.Context, p Profile) error {
 		p.HRZone3MinPct, p.HRZone3MaxPct, p.HRZone4MinPct, p.HRZone4MaxPct,
 		p.HRZone5MinPct, p.HRZone5MaxPct,
 		p.WarmupMinutes, p.CooldownMinutes,
+		p.MinRepresentativePaceSecPerKm, p.MinRepresentativeTimeSeconds,
 	)
 	if err != nil {
 		return fmt.Errorf("update profile: %w", err)

backend/internal/store/profile_test.go

@@ -25,6 +25,9 @@ func TestProfile_DefaultsThenUpdate(t *testing.T) {
 	if p.BackfillHorizonDays != 1095 {
 		t.Errorf("BackfillHorizonDays = %d, want 1095 (migration default)", p.BackfillHorizonDays)
 	}
+	if p.MinRepresentativePaceSecPerKm != 720 || p.MinRepresentativeTimeSeconds != 3 {
+		t.Errorf("pace artifact filter defaults = %+v, want pace=720, time=3", p)
+	}
 
 	maxHR, restingHR := 190.0, 50.0
 	p.GarminEmail = "runner@example.com"
@@ -34,6 +37,8 @@ func TestProfile_DefaultsThenUpdate(t *testing.T) {
 	p.RestingHeartRate = &restingHR
 	p.WarmupMinutes = 8
 	p.BackfillHorizonDays = 14
+	p.MinRepresentativePaceSecPerKm = 600
+	p.MinRepresentativeTimeSeconds = 5
 
 	if err := db.UpdateProfile(ctx, p); err != nil {
 		t.Fatalf("UpdateProfile: %v", err)
@@ -55,4 +60,7 @@ func TestProfile_DefaultsThenUpdate(t *testing.T) {
 	if got.BackfillHorizonDays != 14 {
 		t.Errorf("BackfillHorizonDays = %v, want 14", got.BackfillHorizonDays)
 	}
+	if got.MinRepresentativePaceSecPerKm != 600 || got.MinRepresentativeTimeSeconds != 5 {
+		t.Errorf("pace artifact filter after update = %+v, want pace=600, time=5", got)
+	}
 }