Add GET /tests endpoint returning lightweight test summaries (grade,
score, domain, date) with pagination, using database-level JSON
extraction to avoid loading full report blobs. The feature can be
disabled with --disable-test-list flag. Frontend includes a new
/tests/ page with table view and a conditional "History" navbar link.
Fixes: https://github.com/happyDomain/happydeliver/issues/12
Embed rspamd-symbols.json in the binary to provide human-readable
descriptions for rspamd symbols in reports. Optionally fetch fresh
symbols from a configurable rspamd API URL (--rspamd-api-url flag),
falling back to the embedded list on error. Update the frontend to
display descriptions alongside symbol names and scores.
