refactor: extract serde helpers from stored_object! macro

Move FlexibleIdVisitor, deserialize_flexible_id, and four datetime serde helpers
from repeating inside every macro expansion into a shared
common/src/storage/types/serde_helpers.rs module.

14 macro invocations × 6 items = ~84 fewer redundant function definitions.
Fragile cross-module imports (file_info::deserialize_flexible_id etc.)
are updated to point to the canonical module.

html-router/src/routes/search/handlers.rs

@@ -9,7 +9,8 @@ use axum::{
     response::IntoResponse,
 };
 use common::storage::types::{
-    text_content::{deserialize_flexible_id, TextContent},
+    serde_helpers::deserialize_flexible_id,
+    text_content::TextContent,
     StoredObject,
 };
 use retrieval_pipeline::{RetrievalConfig, SearchResult, SearchTarget, StrategyOutput};