chore(server): improve log messages (#24744)

* Clarify the "asset not found" log during thumbnail generation: it's about database

* Move not found sidecars to verbose level instead of "old=null, new=null" at debug

* Log memory creation at default level

* Add explicit log for missing exif date time

Instead of: Date and time is undefined using exifTag undefined for asset ...

* Log database migration start/end at default level

Currently, these messages are logged as "debug". But they are not printed
when debug or verbose level is set. This is due to the known limitation:
SystemConfigService sets LogLevel later on, after migrations run.

server/src/services/media.service.ts

@@ -158,7 +158,7 @@ export class MediaService extends BaseService {
   async handleGenerateThumbnails({ id }: JobOf<JobName.AssetGenerateThumbnails>): Promise<JobStatus> {
     const asset = await this.assetJobRepository.getForGenerateThumbnailJob(id);
     if (!asset) {
-      this.logger.warn(`Thumbnail generation failed for asset ${id}: not found`);
+      this.logger.warn(`Thumbnail generation failed for asset ${id}: not found in database or missing metadata`);
       return JobStatus.Failed;
     }