feat(workspace): full server + sync implementation, all tests green

- solitaire_server: Axum auth, sync push/pull, leaderboard, daily
  challenge, account deletion, JWT middleware, rate limiting via
  tower_governor, SQLite migrations, health endpoint
- solitaire_server: expose build_test_router (no rate limiting) so
  integration tests work without a peer IP in oneshot requests
- solitaire_sync: SyncPayload, merge logic, shared API types
- solitaire_data: SyncProvider trait, LocalOnlyProvider,
  SolitaireServerClient, auth_tokens keyring integration, blanket
  Box<dyn SyncProvider> impl
- solitaire_data/settings: derive Default on SyncBackend (clippy fix)
- .sqlx/: offline query cache so server compiles without a live DB
- sqlx: removed non-existent "offline" feature flag
- keyring v2: fixed Entry::new() returning Result<Entry>
- sqlx 0.8: all SQLite TEXT columns wrapped in Option<T>
- Integration tests: max_connections(1) on in-memory pool so all
  connections share the same schema

All 191 tests pass; cargo clippy -D warnings clean.

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

solitaire_server/src/challenge.rs

@@ -0,0 +1,115 @@
+//! Daily challenge endpoint.
+//!
+//! `GET /api/daily-challenge` — returns the challenge for today's date.
+//!
+//! The seed is deterministic (same for all players worldwide) and is
+//! generated on first request for that date, then stored in the database
+//! so subsequent calls return the same value.
+
+use axum::{extract::State, Json};
+use chrono::Utc;
+use sqlx::SqlitePool;
+
+use solitaire_sync::ChallengeGoal;
+
+use crate::error::AppError;
+
+// ---------------------------------------------------------------------------
+// Seed generation
+// ---------------------------------------------------------------------------
+
+/// Compute a deterministic seed from a date string such as `"2026-04-26"`.
+///
+/// Uses a simple polynomial rolling hash over the UTF-8 bytes of the string.
+/// The computation is identical across all server instances and all clients
+/// that implement the same algorithm.
+pub fn hash_date_to_u64(date: &str) -> u64 {
+    date.bytes()
+        .fold(0u64, |acc, b| acc.wrapping_mul(31).wrapping_add(b as u64))
+}
+
+/// Generate a [`ChallengeGoal`] from a seed and date.
+///
+/// The goal type and parameters are derived deterministically from the seed
+/// so all players face exactly the same challenge on the same day.
+fn generate_goal(date: &str, seed: u64) -> ChallengeGoal {
+    // Pick a goal variant based on seed modulo number-of-variants.
+    // Three variants cycle through: timed, high-score, and open.
+    match seed % 3 {
+        0 => ChallengeGoal {
+            date: date.to_string(),
+            seed,
+            description: "Win in under 5 minutes".to_string(),
+            target_score: None,
+            max_time_secs: Some(300),
+        },
+        1 => ChallengeGoal {
+            date: date.to_string(),
+            seed,
+            description: "Reach a score of 4 000 or more".to_string(),
+            target_score: Some(4_000),
+            max_time_secs: None,
+        },
+        _ => ChallengeGoal {
+            date: date.to_string(),
+            seed,
+            description: "Win today's deal".to_string(),
+            target_score: None,
+            max_time_secs: None,
+        },
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Database row helper
+// ---------------------------------------------------------------------------
+
+struct ChallengeRow {
+    goal_json: Option<String>,
+}
+
+// ---------------------------------------------------------------------------
+// Handler
+// ---------------------------------------------------------------------------
+
+/// `GET /api/daily-challenge` — no auth required.
+///
+/// Looks up today's challenge in the database. If none exists yet, generates
+/// one deterministically and stores it before returning.
+pub async fn daily_challenge(
+    State(pool): State<SqlitePool>,
+) -> Result<Json<ChallengeGoal>, AppError> {
+    let today = Utc::now().format("%Y-%m-%d").to_string();
+
+    // Try to load an existing row.
+    let row = sqlx::query_as!(
+        ChallengeRow,
+        "SELECT goal_json FROM daily_challenges WHERE date = ?",
+        today
+    )
+    .fetch_optional(&pool)
+    .await?;
+
+    if let Some(r) = row {
+        let json = r.goal_json.ok_or_else(|| AppError::Internal("missing goal_json".into()))?;
+        let goal: ChallengeGoal = serde_json::from_str(&json)?;
+        return Ok(Json(goal));
+    }
+
+    // No row yet — generate and store.
+    let seed = hash_date_to_u64(&today);
+    let goal = generate_goal(&today, seed);
+    let goal_json = serde_json::to_string(&goal)?;
+    let seed_i64 = seed as i64;
+
+    sqlx::query!(
+        "INSERT OR IGNORE INTO daily_challenges (date, seed, goal_json) VALUES (?, ?, ?)",
+        today,
+        seed_i64,
+        goal_json
+    )
+    .execute(&pool)
+    .await?;
+
+    Ok(Json(goal))
+}