perf(server): drop three unused indexes on the sessions table

[otavio]

Stacked on #6883 — based on perf/device-heartbeat-hot-updates so migration numbering
cannot collide. Merge #6883 first; this then targets master on its own.

Migration 020 drops three indexes on sessions that back no filter, no sort and no constraint,
while costing an index insert on every session row written. Over 66 days of production counters
they served 2, 0 and 0 scans, against 204,400 on sessions_started_at_idx.

Index	Scans / 66 d	Size
sessions_username_idx	2	8 MB
sessions_type_idx	0	8 MB
sessions_closed_started_idx	0	42 MB

Unlike the devices pair in #6883 there is no read-path trade to weigh here: nothing queries
these columns, so nothing gets slower. Dropping an index reclaims its space immediately, so no
repack is needed either.

username and type are unreachable, not merely unused

Verified against the source in both shellhub and cloud rather than trusted from counters:

• The session list accepts exactly three filter fields — device_uid, closed, active
  (SessionFilterFields) — and rejects anything else at the route before it reaches SQL.
• The sort is not user-selectable at all: ListSessions hardcodes
  Sorter{By: "started_at", Order: desc, Tiebreak: "id"}. There is no SessionSortFields.
• cloud adds no session filters or sorts of its own.
• No WHERE/ORDER BY on either column exists in either repo. The type = ? predicates that do
  exist are all on session_events, a different table, already served by its session_id/seat
  indexes.

sessions_closed_started_idx is redundant, which is why 0 scans needed explaining

This one does have a consumer, so the counter alone would have been misleading. The
recording-conversion worker runs
WHERE recorded AND closed AND converted = ? ORDER BY started_at DESC LIMIT 10 — close to a
textbook match for (closed, started_at). The query runs on every cron tick; the index is simply
never chosen.

The reason is that its leading column is a near-constant: a session is closed for all but the
minutes it is live, so (closed, started_at) is a strictly fatter duplicate of
sessions_started_at_idx — 42 MB against 26 MB — offering the planner nothing extra for either
the filter or the ORDER BY. The smaller index wins on cost every time, which is exactly what the
204,400-vs-0 split shows.

If that worker ever shows up as a cost, the index it wants is a partial one over the backlog
alone, not this:

CREATE INDEX sessions_pending_conversion_idx ON sessions (started_at DESC)
  WHERE recorded AND closed AND NOT converted;

A few pages instead of 42 MB. Not included here — that would be speculation.

sessions_namespace_id_idx stays

Despite only 63 scans. Deleting a namespace cascades to sessions, and without this index that is
a sequential scan over a 1.19 M-row table. The cascade argument does not depend on idx_scan at
all, so it holds regardless of how shellhub-io/team#200 resolves.

Testing

Full pg store suite green against a schema built from 001 through 020, and verified
end-to-end through bun's runner on the dev stack: 020 applies at boot and leaves sessions with
sessions_pkey, sessions_namespace_id_idx, sessions_device_id_idx and
sessions_started_at_idx.

Fixes shellhub-io/team#199.

[otavio]

Follow-up #6886 is stacked on this branch — it enables pg_stat_statements, pgstattuple and pg_buffercache as migration 021. Merge order: #6883 → #6884 → #6886.