feat: add per-workout-type pace range and expected HR zone

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

backend/internal/store/migrations/0005_workout_type_paces.sql

@@ -0,0 +1,13 @@
+-- Per-workout-type target pace range and expected HR zone. Informational
+-- only: never read by the classification rule engine (see spec section 2).
+-- No history -- overwritten in place when the user updates a value; the
+-- synced activity log is the historical record.
+CREATE TABLE workout_type_paces (
+    workout_kind_id     INTEGER PRIMARY KEY REFERENCES workout_kinds(id),
+    pace_min_sec_per_km REAL,
+    pace_max_sec_per_km REAL,
+    expected_hr_zone    INTEGER CHECK (expected_hr_zone IS NULL OR expected_hr_zone BETWEEN 1 AND 5)
+);
+
+INSERT INTO workout_type_paces (workout_kind_id)
+SELECT id FROM workout_kinds;