fix(api): reject empty ticker in resolve_symbol()

- Change resolve_symbol return type from String to Result<String, IdxError>
- Add early validation for empty/whitespace-only ticker input
- Return Err(IdxError::InvalidInput) for empty tickers
- Add InvalidInput variant to IdxError enum
- Update all 15 callers in cli/stocks.rs to handle Result with ? propagation
- Update tests: empty/whitespace tickers now return error

Fixes #7

src/error.rs

@@ -30,6 +30,8 @@ pub enum IdxError {
     DatabaseError(String),
     #[error("PDF parse error: {0}")]
     PdfParseError(String),
+    #[error("invalid input: {0}")]
+    InvalidInput(String),
 }
 
 #[derive(Debug, Clone, Copy, Serialize, PartialEq, Eq)]
@@ -47,6 +49,7 @@ pub enum ErrorCode {
     AuthError,
     DatabaseError,
     PdfParseError,
+    InvalidInput,
 }
 
 impl IdxError {
@@ -65,6 +68,7 @@ impl IdxError {
             Self::AuthError(_) => ErrorCode::AuthError,
             Self::DatabaseError(_) => ErrorCode::DatabaseError,
             Self::PdfParseError(_) => ErrorCode::PdfParseError,
+            Self::InvalidInput(_) => ErrorCode::InvalidInput,
         }
     }