fix: prevent context canceled errors in web dashboard

Detach DB queries from HTTP request context so HTMX polling doesn't
cancel in-flight queries when the browser aborts previous XHRs. Add
indexes on login_attempts and sessions to speed up frequent dashboard
queries. Bump version to 0.17.1.

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

internal/storage/migrations/005_add_query_indexes.sql

@@ -0,0 +1,3 @@
+CREATE INDEX idx_login_attempts_username ON login_attempts(username);
+CREATE INDEX idx_login_attempts_password ON login_attempts(password);
+CREATE INDEX idx_sessions_disconnected_at ON sessions(disconnected_at);