Phase 7 (Core): chemistry v2, expanded card pool & search filters

- Chemistry v2: FUT-style link scoring with per-player breakdown
  (club +3/link cap 6, league +1/link cap 4, nation +1/link cap 3;
   player cap 10, team cap 100); chemistry response now includes
   per-player breakdown with club/league/nation link counts and pts
- Card pool: 34 new cards across Premier League, La Liga, Bundesliga
  with overlapping clubs/nations for meaningful chemistry testing
- Card search: extended GET /cards with nation, league, club,
  min_overall, max_overall, limit query params; results sorted by
  overall descending
- Match opponent: added "ultimate" difficulty band (85+ OVR);
  random formation selection from 6 tactical formations; falls back
  to lower OVR pool when not enough cards at the requested band
- Tests: 9 new integration tests (28 total, all passing)

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

src/services/match_service.rs

@@ -9,73 +9,70 @@ use crate::{
 };
 use rand::{seq::SliceRandom, Rng};
 
+const FORMATIONS: &[&str] = &[
+    "4-3-3", "4-4-2", "4-2-3-1", "4-1-2-1-2", "3-5-2", "5-3-2",
+];
+
 /// Generate a random AI opponent squad for Squad Battles.
+///
+/// Difficulty bands:
+///   beginner     — overall 55+  (same as "any")
+///   professional — overall 70+
+///   world_class  — overall 78+
+///   legendary    — overall 85+
+///   ultimate     — overall 90+
 pub fn generate_opponent(card_db: &CardDb, difficulty: &str) -> serde_json::Value {
     let (min_overall, names): (u8, &[&str]) = match difficulty {
         "professional" => (
             70,
-            &[
-                "Athletic CF",
-                "City Wanderers",
-                "The Rovers",
-                "United Select",
-                "Blue Stars FC",
-            ],
+            &["Athletic CF", "City Wanderers", "The Rovers", "United Select", "Blue Stars FC"],
         ),
         "world_class" => (
             78,
-            &[
-                "Elite Stars FC",
-                "Champions Select",
-                "Premier XI",
-                "Galaxy United",
-                "Titan FC",
-            ],
+            &["Elite Stars FC", "Champions Select", "Premier XI", "Galaxy United", "Titan FC"],
         ),
         "legendary" => (
             85,
-            &[
-                "Legends United",
-                "Ultimate XI",
-                "Gold Standard FC",
-                "The Icons",
-                "Heritage FC",
-            ],
+            &["Legends United", "Ultimate XI", "Gold Standard FC", "The Icons", "Heritage FC"],
+        ),
+        "ultimate" => (
+            90,
+            &["Apex XI", "Pantheon FC", "Gods of FUT", "Invincibles Select", "Eternal XI"],
         ),
         _ => (
-            58,
-            &[
-                "Amateur Town FC",
-                "Sunday League XI",
-                "Park FC",
-                "Village Stars",
-                "Reserve XI",
-            ],
+            55,
+            &["Amateur Town FC", "Sunday League XI", "Park FC", "Village Stars", "Reserve XI"],
         ),
     };
 
     let mut rng = rand::thread_rng();
     let all = card_db.by_min_overall(min_overall);
-    let mut indices: Vec<usize> = (0..all.len()).collect();
+
+    // Fall back to lower overall band if not enough cards at this difficulty
+    let pool: Vec<_> = if all.len() >= 11 {
+        all
+    } else {
+        card_db.by_min_overall(55)
+    };
+
+    let mut indices: Vec<usize> = (0..pool.len()).collect();
     indices.shuffle(&mut rng);
-    let cards: Vec<_> = indices
-        .into_iter()
-        .take(11)
-        .map(|i| all[i].clone())
-        .collect();
+    let cards: Vec<_> = indices.into_iter().take(11).map(|i| pool[i].clone()).collect();
 
     let squad_rating = if cards.is_empty() {
         0
     } else {
         cards.iter().map(|c| c.overall as i64).sum::<i64>() / cards.len() as i64
     };
+
     let name = names[rng.gen_range(0..names.len())];
+    let formation = FORMATIONS[rng.gen_range(0..FORMATIONS.len())];
 
     serde_json::json!({
         "opponent_name": name,
         "difficulty": difficulty,
         "squad_rating": squad_rating,
-        "formation": "4-4-2",
+        "formation": formation,
         "cards": cards,
     })
 }