refactor: Phase A — extraction des helpers partagés et micro-fixes

- Centralise remap_libraries_path/unmap_libraries_path dans crates/core/paths.rs
  (supprime 4 copies dupliquées dans API + indexer)
- Centralise mode_to_interval_minutes/validate_schedule_mode dans crates/core/schedule.rs
  (remplace 8 match blocks + 4 validations inline)
- Ajoute helpers env_or<T>/env_string_or dans config.rs, utilise ThumbnailConfig::default()
  comme base dans from_env() (élimine la duplication des valeurs par défaut)
- Supprime std::mem::take inutile dans books.rs
- Cible #[allow(dead_code)] sur le champ plutôt que le struct (metadata.rs)
- Remplace eprintln! par tracing::warn! dans parsers
- Fix clippy boolean logic bug dans prowlarr.rs

10 nouveaux tests unitaires (paths + schedule)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/core/src/schedule.rs

@@ -0,0 +1,57 @@
+/// Valid schedule modes for library monitoring.
+pub const VALID_SCHEDULE_MODES: &[&str] = &["manual", "hourly", "daily", "weekly"];
+
+/// Convert a schedule mode to its interval in minutes.
+/// Returns the interval for known modes, defaults to daily (1440) for unknown values.
+pub fn mode_to_interval_minutes(mode: &str) -> i64 {
+    match mode {
+        "hourly" => 60,
+        "daily" => 1440,
+        "weekly" => 10080,
+        _ => 1440,
+    }
+}
+
+/// Validate that a mode string is one of the accepted schedule modes.
+/// Returns an error message if invalid.
+pub fn validate_schedule_mode(mode: &str) -> Result<(), &'static str> {
+    if VALID_SCHEDULE_MODES.contains(&mode) {
+        Ok(())
+    } else {
+        Err("must be one of: manual, hourly, daily, weekly")
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn interval_known_modes() {
+        assert_eq!(mode_to_interval_minutes("hourly"), 60);
+        assert_eq!(mode_to_interval_minutes("daily"), 1440);
+        assert_eq!(mode_to_interval_minutes("weekly"), 10080);
+    }
+
+    #[test]
+    fn interval_unknown_defaults_to_daily() {
+        assert_eq!(mode_to_interval_minutes("manual"), 1440);
+        assert_eq!(mode_to_interval_minutes("unknown"), 1440);
+        assert_eq!(mode_to_interval_minutes(""), 1440);
+    }
+
+    #[test]
+    fn validate_accepts_valid() {
+        assert!(validate_schedule_mode("manual").is_ok());
+        assert!(validate_schedule_mode("hourly").is_ok());
+        assert!(validate_schedule_mode("daily").is_ok());
+        assert!(validate_schedule_mode("weekly").is_ok());
+    }
+
+    #[test]
+    fn validate_rejects_invalid() {
+        assert!(validate_schedule_mode("monthly").is_err());
+        assert!(validate_schedule_mode("").is_err());
+        assert!(validate_schedule_mode("HOURLY").is_err());
+    }
+}