chore: remove deprecated demo make target and update attachment thumbnail resolution

- Remove `make demo` target from Makefile, AGENTS.md, and README.md
- Improve `_row_to_attachment` to glob for thumbnail files with any extension instead of hardcoding `.jpg`
- Rewrite `cmd_attachments_prune` to use time-based cutoff and `delete_attachment` helper instead of orphan detection
- Add cursor-based pagination to feed and notifications endpoints with `before` parameter
- Track `did_upvote` flag in vote handler to only notify on new upvotes, not vote changes
- Add `PAGE_SIZE` constant and `next_cursor` template variable to notifications page
- Implement Playwright tests for notification pagination with 30 seeded notifications

Makefile

@@ -7,7 +7,7 @@ LOCUST_SPAWN_RATE ?= 5
 LOCUST_RUN_TIME ?= 120s
 DEVPLACE_RATE_LIMIT ?= 1000000
 
-.PHONY: install dev clean test test-headed demo locust locust-headless
+.PHONY: install dev clean test test-headed locust locust-headless
 
 install:
 	pip install -e .
@@ -24,9 +24,6 @@ test:
 test-headed:
 	PLAYWRIGHT_HEADLESS=0 python -m pytest tests/ -v --tb=line -x
 
-demo:
-	PLAYWRIGHT_HEADLESS=0 python -m pytest tests/test_demo.py -v -s --tb=line -x
-
 locust:
 	export DEVPLACE_DATABASE_URL="sqlite:///$(LOCUST_DB)"; \
 	export DEVPLACE_RATE_LIMIT=$(DEVPLACE_RATE_LIMIT); \