feat(timeline): compute a letter's linkedEventId in TimelineService

For each LETTER entry, resolve the curated event whose documents set
contains the letter, in one batched pass over the events already loaded
(no per-letter query, no new column). The DTO gains a nullable
linkedEventId; non-letter entries keep null.

Refs #850

frontend/src/lib/generated/api.ts

@@ -2467,6 +2467,8 @@ export interface components {
             rootTagId?: string;
             rootTagName?: string;
             rootTagColor?: string;
+            /** Format: uuid */
+            linkedEventId?: string;
         };
         TimelineYearDTO: {
             /** Format: int32 */