fix: inline DB error source into DbError struct

The previous fix logged the underlying error in a separate log line,
but the error that propagated up still showed just "DbError { kind:
InsertError }" at the call site. Now the source message is captured
on the struct itself, so Debug/Display output at any call site shows
the actual Diesel error inline.

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

src/database/mod.rs

@@ -193,19 +193,26 @@ pub fn connect() -> SqliteConnection {
     conn
 }
 
-#[derive(Debug)]
 pub struct DbError {
     pub kind: DbErrorKind,
+    pub source: Option<String>,
 }
 
 impl DbError {
     fn new(kind: DbErrorKind) -> Self {
-        DbError { kind }
+        DbError { kind, source: None }
     }
 
+    /// Capture the source error message AND log it. Callers should use
+    /// this from `map_err` closures so the underlying Diesel/SQLite
+    /// error survives the conversion to `DbError`.
     fn log(kind: DbErrorKind, source: impl std::fmt::Display) -> Self {
-        log::error!("DB {:?}: {}", kind, source);
-        DbError { kind }
+        let msg = source.to_string();
+        log::error!("DB {:?}: {}", kind, msg);
+        DbError {
+            kind,
+            source: Some(msg),
+        }
     }
 
     fn exists() -> Self {
@@ -213,6 +220,26 @@ impl DbError {
     }
 }
 
+impl std::fmt::Debug for DbError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match &self.source {
+            Some(s) => write!(f, "DbError {{ kind: {:?}, source: {} }}", self.kind, s),
+            None => write!(f, "DbError {{ kind: {:?} }}", self.kind),
+        }
+    }
+}
+
+impl std::fmt::Display for DbError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match &self.source {
+            Some(s) => write!(f, "{:?}: {}", self.kind, s),
+            None => write!(f, "{:?}", self.kind),
+        }
+    }
+}
+
+impl std::error::Error for DbError {}
+
 #[derive(Debug, PartialEq)]
 pub enum DbErrorKind {
     AlreadyExists,

src/testhelpers.rs

@@ -144,6 +144,7 @@ impl PreviewDao for TestPreviewDao {
         } else {
             Err(DbError {
                 kind: DbErrorKind::UpdateError,
+                source: None,
             })
         }
     }