test(microformats): Add v1.3.0 validation tests for tags and h-feed

Phase 4: Validation per microformats-tags-design.md

Added test fixtures:
- published_note_with_tags: Creates note with test tags for p-category validation
- published_note_with_media: Creates note with media for u-photo placement testing

Added v1.3.0 microformats2 validation tests:
- test_hfeed_has_required_properties: Validates name, author, url per spec
- test_hfeed_author_is_valid_hcard: Validates h-card structure
- test_hentry_has_pcategory_for_tags: Validates p-category markup
- test_uphoto_outside_econtent: Validates u-photo placement per draft spec

Test results:
- All 18 microformats tests pass
- All 116 related tests pass (microformats, notes, micropub)
- Confirms Phases 1-3 implementation correctness

Updated BACKLOG.md with tag-filtered feeds feature (medium priority)

Implementation report: docs/design/v1.3.0/2025-12-10-phase4-implementation.md

Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docs/projectplan/BACKLOG.md

@@ -45,6 +45,12 @@
 
 ## Medium
 
+### Tag-Filtered Feeds
+- Filter feeds by tag (e.g., `/feed.rss?tag=python`)
+- Dedicated tag feed URLs (e.g., `/tags/python/feed.rss`)
+- Support all three formats (RSS, Atom, JSON Feed)
+- Cache management for filtered feeds
+
 ### Webmentions
 - Receive endpoint
 - Send on publish