Add yes/no RSVP answers with required login

RSVPs now carry a status ('yes'/'no'). When auth is enabled, answering
requires login for all events, not just capped ones. Answers are deduped
one per user and re-answering updates in place. A 'no' zeroes plus_one,
is excluded from Going counts and cap math, frees the person's slot
claims, and shows in a separate "Can't make it" list.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

schema.sql

@@ -35,6 +35,7 @@ CREATE TABLE IF NOT EXISTS rsvps (
     name       TEXT    NOT NULL,
     note       TEXT    NOT NULL DEFAULT '',
     plus_one   INTEGER NOT NULL DEFAULT 0,
+    status     TEXT    NOT NULL DEFAULT 'yes', -- 'yes' or 'no'
     user_id    INTEGER REFERENCES users(id),
     created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
 );