Add RSVP support and Open Graph meta tags

People can now RSVP without claiming a slot. OG tags show
event title, date/time/location, and headcount in link previews.

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

schema.sql

@@ -26,5 +26,14 @@ CREATE TABLE IF NOT EXISTS claims (
     created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
 );
 
+CREATE TABLE IF NOT EXISTS rsvps (
+    id         INTEGER PRIMARY KEY AUTOINCREMENT,
+    event_id   INTEGER NOT NULL REFERENCES events(id) ON DELETE CASCADE,
+    name       TEXT    NOT NULL,
+    note       TEXT    NOT NULL DEFAULT '',
+    created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
 CREATE INDEX IF NOT EXISTS idx_slots_event ON slots(event_id);
 CREATE INDEX IF NOT EXISTS idx_claims_slot ON claims(slot_id);
+CREATE INDEX IF NOT EXISTS idx_rsvps_event ON rsvps(event_id);