feat(alertd): cap alert SQL result at 100 rows

[passcod]

🤖 Some alerts run SQL that returns a large number of rows; loading all of them into memory is wasteful when an alert only needs to know it fired and to template a sample.

This caps an SQL alert source at 100 rows, bounded at the database portal so excess rows are never pulled into memory:

• The SQL source now opens a read transaction, binds the prepared statement to a portal, and fetches via query_portal with a ROW_LIMIT of 100, rather than buffering the entire result with query.
• When a result hits the cap, a warning is logged (with the alert file and the limit) so operators know truncation happened.
• Numerical thresholds and the rows template context therefore operate on at most 100 rows.

Adds a test asserting a 1000-row generate_series source is capped at 100.