Phase 10: card upgrade system (chemistry styles, position change, training)

- GET /chemistry-styles — lists 18 FUT-style chemistry styles with stat boost breakdowns
- POST /collection/:id/chemistry-style — apply a style (Shadow, Hunter, Anchor, etc.)
- POST /collection/:id/position — override a player's position for 500 coins
- POST /collection/:id/training — add +1/+2/+3 OVR training bonus (max +3 total)
- GET /collection now includes chemistry_style, position_override, training_bonus,
  effective_overall and effective_position fields per owned card
- Migration 0007 adds three columns to owned_cards with safe defaults
- 10 new integration tests — all 55 pass, clippy clean

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/models/card.rs

@@ -42,4 +42,7 @@ pub struct OwnedCard {
     pub is_loan: bool,
     pub loan_matches_remaining: Option<i64>,
     pub acquired_at: String,
+    pub chemistry_style: String,
+    pub position_override: Option<String>,
+    pub training_bonus: i64,
 }