chore: lowercase all error messages and add # Errors doc sections

- Fix err-lowercase-msg: normalize all #[error(...)] display strings to
  lowercase (AppError, FileError, ApiErr) and update affected tests
- Fix err-doc-errors: add # Errors sections to 25+ fallible public
  functions across db.rs, store.rs, embedding.rs, indexes.rs,
  ingestion_task.rs, and ingest_limits.rs

common/src/utils/ingest_limits.rs

@@ -6,6 +6,15 @@ pub enum IngestValidationError {
     BadRequest(String),
 }
 
+/// Validates ingestion input against configured limits.
+///
+/// Checks file count, content size, context size, and category length.
+///
+/// # Errors
+///
+/// Returns `IngestValidationError::BadRequest` if the file count exceeds the maximum.
+/// Returns `IngestValidationError::PayloadTooLarge` if content, context, or
+/// category exceed their configured byte limits.
 pub fn validate_ingest_input(
     config: &AppConfig,
     content: Option<&str>,