feat(gamification): list_recent_completions query + DailyCompletionCount

Returns a fixed-length, oldest-first series of daily completion counts
for the last N local-time days. Excludes cascade parents and
uncompleted rows. Empty days are explicit zeros, not missing entries,
so the Phase 8 sparkline can render a fixed 7 bars.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/storage/Cargo.toml

@@ -18,6 +18,9 @@ sqlx = { version = "0.8", default-features = false, features = ["runtime-tokio",
 # Async runtime
 tokio = { version = "1", features = ["rt", "sync", "macros"] }
 
+# Serialisation (DailyCompletionCount exposed to frontend via Tauri commands)
+serde = { version = "1", features = ["derive"] }
+
 # Logging
 log = "0.4"