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/memory.service.ts

@@ -28,6 +28,7 @@ export class MemoryService extends BaseService {
           continue;
         }
 
+        this.logger.log(`Creating memories for ${target.toISO()}`);
         try {
           await Promise.all(users.map((owner) => this.createOnThisDayMemories(owner.id, target)));
         } catch (error) {