perf(jobs): endpoint search does a sequential scan (consider pg_trgm GIN index)

[tsdk02]

Summary

The jobs-list endpoint search (?endpoint=..., added in #35) matches with a
leading-wildcard ILIKE '%' || $n || '%'. A leading % cannot use a B-tree
index, so this is a sequential scan of the workspace's jobs table on every
filtered request. Combined filters beyond status (which has
idx_jobs_status) also have no supporting index.

This is fine at current data volumes and was intentionally deferred from the
pagination/filter hardening PR — filing it so the perf work isn't lost.

Where

• crates/common/src/db/jobs.rs — build_list_query, the endpoint ILIKE ...
  condition.
• Per-workspace schema template: migrations/workspace_v1.sql.

Proposed fix

Add a trigram GIN index so leading-wildcard ILIKE can be index-assisted:

CREATE EXTENSION IF NOT EXISTS pg_trgm;
CREATE INDEX IF NOT EXISTS idx_{p}jobs_endpoint_trgm
    ON {p}jobs USING gin (endpoint gin_trgm_ops);

Considerations:

• Requires the pg_trgm extension to be available/enabled in each tenant
  schema's database; add to the workspace provisioning template.
• Measure first — for small per-workspace tables the planner may still prefer a
  seq scan, and the index has write-amplification cost.

Acceptance

• Decide whether endpoint search volume justifies the index.
• If yes, add pg_trgm + GIN index to workspace_v1.sql and verify
  EXPLAIN shows it used for a representative ILIKE '%x%' query.

Follow-up to #35.