feat: add CI workflow, clean target, and fix user_stats view with deduplicated columns

Add .gitea/workflows/export.yaml for automated build on push, introduce `clean` target in Makefile to recreate export directory, remove stale `dump_text.py` from SOURCES.txt, and correct duplicate column definitions in the `user_stats` SQL view creation inside db.py.

Makefile

@@ -1,4 +1,4 @@
-all: build sync_excempt export_dataset export_stats merge_images
+all: build clean sync_excempt export_dataset export_stats merge_images
 
 build:
 	time pip install build 
@@ -10,6 +10,10 @@ build:
 sync:
 	time dr.sync
 
+clean:
+	-@rm -r export
+	mkdir -p export 
+
 sync_excempt:
 	@echo "Sync is not executed because it's a lengthy process ending with timeout error."