feat: reseed workout_kinds with the fixed 7-type running taxonomy

- Add migration 0004_workout_taxonomy.sql to reset workout_kinds table
  with exactly 7 fixed named kinds (Easy Run, Long Run, Threshold 30',
  Threshold 60', Tempo, Interval, MAS Test), each with a never-matching
  placeholder rule
- Add test TestWorkoutTaxonomy_SeededWithSevenFixedTypes to verify all
  7 kinds are seeded and active
- Update TestKindAssignment_AppendOnlyHistoryAndCurrentView to use a
  unique kind name to avoid conflict with seeded kinds

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

backend/internal/store/migrations/0004_workout_taxonomy.sql

@@ -0,0 +1,17 @@
+-- The taxonomy is now a fixed, closed set (no more user-created "kinds").
+-- Any prior arbitrary kinds and their classification history are reset:
+-- the concept they classified against no longer exists.
+DELETE FROM kind_assignments;
+DELETE FROM workout_kinds;
+
+-- Placeholder rule: distance is never negative, so this never matches.
+-- Every activity starts in needs_review for every type until real rules
+-- are tuned (a follow-up plan, not this migration).
+INSERT INTO workout_kinds (name, description, color, rule_json, priority, is_active) VALUES
+    ('Easy Run',       '', '#22c55e', '{"match":"all","conditions":[{"metric":"distance_meters","op":"<","value":0}]}', 0, 1),
+    ('Long Run',       '', '#3b82f6', '{"match":"all","conditions":[{"metric":"distance_meters","op":"<","value":0}]}', 0, 1),
+    ('Threshold 30''', '', '#f59e0b', '{"match":"all","conditions":[{"metric":"distance_meters","op":"<","value":0}]}', 0, 1),
+    ('Threshold 60''', '', '#f59e0b', '{"match":"all","conditions":[{"metric":"distance_meters","op":"<","value":0}]}', 0, 1),
+    ('Tempo',          '', '#eab308', '{"match":"all","conditions":[{"metric":"distance_meters","op":"<","value":0}]}', 0, 1),
+    ('Interval',       '', '#ef4444', '{"match":"all","conditions":[{"metric":"distance_meters","op":"<","value":0}]}', 0, 1),
+    ('MAS Test',       '', '#a855f7', '{"match":"all","conditions":[{"metric":"distance_meters","op":"<","value":0}]}', 0, 1);