Create compound index for vulnerabilities table on project_id, severity & state

This is a followup issue for !36040 (merged). Please see the discussion here: !36040 (comment 376967397).

We can also create a partial index only for states (1, 4) which will make the index size smaller.

Implementation plan

• backend add index on project_id, state and severity to vulnerabilities table:

CREATE INDEX index_vulnerabilities_on_project_id_state_and_severity ON public.vulnerabilities USING btree (project_id, state, severity);

• backend consider creating partial index instead to cover only 1 and 4 states (detected, confirmed)