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).

frontend/src/types/api.ts

@@ -113,6 +113,8 @@ export interface Profile {
   HRZone5MaxPct: number;
   WarmupMinutes: number;
   CooldownMinutes: number;
+  MinRepresentativePaceSecPerKm: number;
+  MinRepresentativeTimeSeconds: number;
   CreatedAt: string;
   UpdatedAt: string;
 }