test(db): pin the query plan for the monthly aggregates

[argszero]

Summary

Pin the query plan for the monthly aggregates, completing acceptance item 2 of the perf rant
(2026-09-14T16:51:14): "对月度聚合 EXPLAIN QUERY PLAN 不得出现 SCAN transactions".

#234 fixed the predicates and added the indexes, and perf_gate asserts the source shape (no date
function wraps a time column). But shape alone is not enough: if the covering indexes were dropped,
the source would still look right and the suite would stay green. This PR adds the plan-level half.

Related Issue

Follow-up acceptance item of rant 2026-09-14T16:51:14; no GitHub issue exists (left empty rather
than fabricating one).

Changes

• src/db.rs: new db::tests::month_aggregate_queries_use_an_index_not_a_full_scan. On a migrated db
  with 3000 rows spread over the last 180 days (so the month window is ~1/6 of the table and the index
  is genuinely attractive), each rewritten shape must show SEARCH <table> and must not show
  SCAN <table>:
  • per-user month consume (wallet), global month-in (ops, no user_id), monthly usage count (ops),
    today-by-hour (ops), per-member LEFT JOIN (admin).
  • the admin case asserts on the alias ur - that is what the planner prints for an aliased
    table; asserting the table name there is a false failure (found and fixed while writing this).
• Records the rant's open question on index de-duplication: the new transactions(user_id, time, type, pts)
  has transactions(user_id, time) as a prefix, so the older idx_transactions_user_id_time is
  prefix-redundant. Deliberately not dropped here (minimal change) - a follow-up candidate.

Honest scope note: this test pins the plan permitted for the range shape; production is bound to
that shape by perf_gate. Neither half alone is sufficient - together they assert "production has the
shape" and "the shape gets a SEARCH plan".

• Config/data-structure changes are mirrored into the example file (n/a - test-only)

Tests

• cargo test all green - 246 passed / 0 failed (was 245; +1)
• cargo fmt --check exit 0
• cargo clippy --all-targets -- -D warnings clean on CI's stable toolchain (same pre-existing
  1.95.0-only collapsible_match disclosure as before, out of this diff)
• New test added (1)

Teeth

Removing the two new covering indexes (idx_transactions_user_id_time_type_pts,
idx_transactions_time_type_pts) from migrate() reds:

db::tests::month_aggregate_queries_use_an_index_not_a_full_scan
db::tests::transactions_perf_indexes_created_on_migrate

and the file was restored byte-identically (md5 asserted before/after). The live tree is green.

Checklist

• Branch naming follows the convention: test/month-aggregate-query-plan
• Commit message uses Conventional Commits
• Single responsibility, minimal change