feat: Phase 2 — game feel complete

Chemistry & squads:
- Chemistry calculation on GET /squad (club/league/nation links, max 100)
- Formation validation on POST /squad (exactly 11 starters, exactly 1 GK)

Objectives:
- Weekly objectives JSON (4 objectives: warrior, goals, dedicated, SBC)
- Daily objectives auto-reset at midnight UTC (background task)

SBC validation expanded:
- max_overall per-player enforcement
- required_clubs validation
- min_players_from_same_nation validation
- min_players_from_same_club validation

Market:
- GET /market?min_overall=X&position=Y filtering
- Expiry cleanup runs before every NPC refresh
- NPC market auto-refresh every 24h (background task, runs at startup)

Matches:
- GET /matches/opponent?difficulty=beginner|professional|world_class|legendary
  generates a random AI opponent squad from the card pool

Statistics:
- win_streak and best_win_streak tracking (migration 0002)
- GET /statistics/history?limit=N — last N matches with summary stats

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

src/services/match_service.rs

@@ -5,8 +5,80 @@ use crate::{
         match_result::{Match, MatchRewardResult, SubmitMatchRequest},
         objective::ObjectiveDefinition,
     },
-    services::{club, objective, profile, statistics},
+    services::{card_db::CardDb, club, objective, profile, statistics},
 };
+use rand::{seq::SliceRandom, Rng};
+
+/// Generate a random AI opponent squad for Squad Battles.
+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",
+            ],
+        ),
+        "world_class" => (
+            78,
+            &[
+                "Elite Stars FC",
+                "Champions Select",
+                "Premier XI",
+                "Galaxy United",
+                "Titan FC",
+            ],
+        ),
+        "legendary" => (
+            85,
+            &[
+                "Legends United",
+                "Ultimate XI",
+                "Gold Standard FC",
+                "The Icons",
+                "Heritage FC",
+            ],
+        ),
+        _ => (
+            58,
+            &[
+                "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();
+    indices.shuffle(&mut rng);
+    let cards: Vec<_> = indices
+        .into_iter()
+        .take(11)
+        .map(|i| all[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())];
+
+    serde_json::json!({
+        "opponent_name": name,
+        "difficulty": difficulty,
+        "squad_rating": squad_rating,
+        "formation": "4-4-2",
+        "cards": cards,
+    })
+}
 
 const COINS_WIN: i64 = 400;
 const COINS_DRAW: i64 = 150;