feat(tags): Add tag archive route and admin interface integration

Implement Phase 3 of v1.3.0 tags feature per microformats-tags-design.md:

Routes (starpunk/routes/public.py):
- Add /tag/<tag> archive route with normalization and 404 handling
- Pre-load tags in index route for all notes
- Pre-load tags in note route for individual notes

Admin (starpunk/routes/admin.py):
- Parse comma-separated tag input in create route
- Parse tag input in update route
- Pre-load tags when displaying edit form
- Empty tag field removes all tags

Templates:
- Add tag input field to templates/admin/edit.html
- Add tag input field to templates/admin/new.html
- Use Jinja2 map filter to display existing tags

Implementation details:
- Tag URL parameter normalized to lowercase before lookup
- Tags pre-loaded using object.__setattr__ pattern (like media)
- parse_tag_input() handles trim, dedupe, normalization
- All existing tests pass (micropub categories, admin routes)

Per architect design:
- No pagination on tag archives (acceptable for v1.3.0)
- No autocomplete in admin (out of scope)
- Follows existing media loading patterns

Generated with Claude Code

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

templates/admin/new.html

@@ -51,6 +51,15 @@
     <!-- Preview area (filled via JavaScript after file selection) -->
     <div id="media-preview" class="media-preview" style="display: none;"></div>
 
+    <div class="form-group">
+      <label for="tags">Tags</label>
+      <input type="text"
+             id="tags"
+             name="tags"
+             placeholder="Comma-separated tags (e.g., IndieWeb, Python, Thoughts)">
+      <small>Separate multiple tags with commas</small>
+    </div>
+
     <div class="form-group form-checkbox">
       <input type="checkbox" id="published" name="published" checked>
       <label for="published">Publish immediately</label>