Enable cross-subdomain htmx and purify layout to sexp

- Disable htmx selfRequestsOnly, add CORS headers for *.rose-ash.com
- Remove same-origin guards from ~menu-row and ~nav-link htmx attrs
- Convert ~app-layout from string-concatenated HTML to pure sexp tree
- Extract ~app-head component, replace ~app-shell with inline structure
- Convert hamburger SVG from Python HTML constant to ~hamburger sexp component
- Fix cross-domain fragment URLs (events_url, market_url)
- Fix starts-with? primitive to handle nil values
- Fix duplicate admin menu rows on OOB swaps
- Add calendar admin nav links (slots, description)
- Convert slots page from Jinja to sexp rendering
- Disable page caching in development mode
- Backfill migration to clean orphaned container_relations

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

relations/alembic/versions/0003_backfill_calendar_metadata.py

@@ -0,0 +1,30 @@
+"""Backfill label + metadata for calendar relations.
+
+Removes calendar relations with no metadata slug and no label,
+which are either orphaned or were created before metadata was tracked.
+
+Revision ID: relations_0003
+Revises: relations_0002
+Create Date: 2026-02-28
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = "relations_0003"
+down_revision = "relations_0002"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.execute("""
+        DELETE FROM container_relations
+        WHERE child_type IN ('calendar', 'market')
+          AND (metadata IS NULL OR metadata->>'slug' IS NULL)
+          AND label IS NULL
+    """)
+
+
+def downgrade() -> None:
+    pass