feat(seed): add API test-data seeding script and make testdata target

- scripts/seed.py: stdlib HTTP client, JPEG+EXIF via optional Pillow (PNG fallback), additive-only, dry-run
- Makefile: testdata target with ALBUMS/PHOTOS/SEED/TIMEOUT and boolean flags
- README: usage section; new dev-seed-data capability spec

Makefile

@@ -1,8 +1,16 @@
-.PHONY: build build-release frontend backend run migrate reconcile reconcile-delete tidy test clean
+.PHONY: build build-release frontend backend run migrate reconcile reconcile-delete tidy test testdata clean
 
 # Release version stamped into the binary (-X main.version). Override with VERSION=...
 VERSION ?= $(shell git describe --tags --always --dirty 2>/dev/null || echo dev)
 
+# Seed testdata parameters (see `make testdata`). Counts: ALBUMS, PHOTOS, SEED, TIMEOUT.
+# Boolean (set to 1 to enable): TAGS, COMMENTS, LIKES, SHARE, INVITES, WAIT, DRY.
+# Extra passthrough flags: ARGS.
+ALBUMS ?= 2
+PHOTOS ?= 10
+SEED ?= 0
+TIMEOUT ?= 30
+
 frontend:
 	cd frontend && npm ci && npm run build
 	rm -rf backend/static/*
@@ -53,6 +61,25 @@ tidy:
 test:
 	cd backend && go test ./...
 
+# Seed test data into a running instance through the public API (additive only).
+# Credentials come from the environment: GALO_EMAIL, GALO_PASSWORD
+# (optional GALO_BASE_URL). Example:
+#   make testdata ALBUMS=3 PHOTOS=5 TAGS=1 LIKES=1 SEED=42
+testdata:
+	@python3 scripts/seed.py \
+		--albums $(ALBUMS) \
+		--photos-per-album $(PHOTOS) \
+		--seed $(SEED) \
+		--timeout $(TIMEOUT) \
+		$(if $(filter 1,$(TAGS)),--tags,) \
+		$(if $(filter 1,$(COMMENTS)),--comments,) \
+		$(if $(filter 1,$(LIKES)),--likes,) \
+		$(if $(filter 1,$(SHARE)),--share-links,) \
+		$(if $(filter 1,$(INVITES)),--invites,) \
+		$(if $(filter 1,$(WAIT)),--wait-variants,) \
+		$(if $(filter 1,$(DRY)),--dry-run,) \
+		$(ARGS)
+
 clean:
 	rm -rf bin backend/static/*
 	touch backend/static/.gitkeep