obs,sql,server,ui: add statement store and V2 SQL Activity endpoints

[kyle-a-wong]

Summary

Add a new statement fingerprint store and V2 SQL Activity API endpoints
with server-side pagination, sorting, and filtering. This replaces the
expensive client-side aggregation in the existing gRPC endpoints with
efficient server-side queries against the denormalized covering indexes
on system.statement_statistics and system.transaction_statistics.

• Statement store (pkg/obs/statementstore): Write-through FIFO
  cache that deduplicates and asynchronously persists statement
  fingerprints to system.statements via a background writer goroutine.
• Ingester integration: The SQL stats ingester calls the store on
  each statement observation, including when fingerprint IDs are
  recomputed due to implicit/explicit transaction changes.
• Backfill migration: A v26.3 cluster upgrade backfills
  system.statements from system.statement_statistics so fingerprints
  observed before the store existed are immediately available.
• V2 BFF endpoints: GET /api/v2/dbconsole/statements/ and
  GET /api/v2/dbconsole/transactions/ with server-side pagination,
  sorting, time range, search, app name, database, and internal
  statement filters. Datadriven EXPLAIN tests verify covering index
  usage.
• UI pages: StatementsPageV2 and TransactionsPageV2 React
  components using SWR hooks, accessible from the Advanced Debug panel.

The individual commits speak for themselves — each is a self-contained
logical unit that builds on the previous one.

Epic: none

Release note: None

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[cockroach-teamcity]

This change is 

[cockroach-teamcity]

⚪ Sysbench [SQL, 3node, oltp_read_write]

Metric	Old Commit	New Commit	Delta	Note
⚪ sec/op	11.04m ±9%	10.95m ±8%	~	p=0.902 n=15
⚪ allocs/op	8.128k ±0%	8.175k ±1%	~	p=0.191 n=15

Reproduce

benchdiff binaries:

mkdir -p benchdiff/2944efe/bin/1058449141
gcloud storage cp gs://cockroach-microbench-ci/builds/2944efe978a7b7eaa17826772f9d4ca633b6f75a/bin/pkg_sql_tests benchdiff/2944efe/bin/1058449141/cockroachdb_cockroach_pkg_sql_tests
chmod +x benchdiff/2944efe/bin/1058449141/cockroachdb_cockroach_pkg_sql_tests
mkdir -p benchdiff/3944370/bin/1058449141
gcloud storage cp gs://cockroach-microbench-ci/builds/3944370be734756fdc54b5aa04934f6c4315464c/bin/pkg_sql_tests benchdiff/3944370/bin/1058449141/cockroachdb_cockroach_pkg_sql_tests
chmod +x benchdiff/3944370/bin/1058449141/cockroachdb_cockroach_pkg_sql_tests

benchdiff command:

# NB: for best (most stable) results, also add a suitable `--benchtime` that
# results in ~1s to ~5s of benchmark runs. For example, if ops average ~3ms, a
# benchtime of `1000x` is appropriate.
#
# Some benchmarks (in particular BenchmarkSysbench) output additional memory
# profiles covering only the execution (excluding the setup/teardown) - those
# should be preferred for analysis since they more closely correspond to what's
# reported as B/op and alloc/op.
benchdiff --run=^BenchmarkSysbench/SQL/3node/oltp_read_write$ --old=3944370 --new=2944efe --memprofile ./pkg/sql/tests

⚪ Sysbench [KV, 3node, oltp_read_only]

Metric	Old Commit	New Commit	Delta	Note
⚪ sec/op	3.572m ±10%	3.569m ±11%	~	p=0.567 n=15
⚪ allocs/op	2.108k ±0%	2.109k ±0%	~	p=0.120 n=15

Reproduce

benchdiff binaries:

mkdir -p benchdiff/2944efe/bin/1058449141
gcloud storage cp gs://cockroach-microbench-ci/builds/2944efe978a7b7eaa17826772f9d4ca633b6f75a/bin/pkg_sql_tests benchdiff/2944efe/bin/1058449141/cockroachdb_cockroach_pkg_sql_tests
chmod +x benchdiff/2944efe/bin/1058449141/cockroachdb_cockroach_pkg_sql_tests
mkdir -p benchdiff/3944370/bin/1058449141
gcloud storage cp gs://cockroach-microbench-ci/builds/3944370be734756fdc54b5aa04934f6c4315464c/bin/pkg_sql_tests benchdiff/3944370/bin/1058449141/cockroachdb_cockroach_pkg_sql_tests
chmod +x benchdiff/3944370/bin/1058449141/cockroachdb_cockroach_pkg_sql_tests

benchdiff command:

# NB: for best (most stable) results, also add a suitable `--benchtime` that
# results in ~1s to ~5s of benchmark runs. For example, if ops average ~3ms, a
# benchtime of `1000x` is appropriate.
#
# Some benchmarks (in particular BenchmarkSysbench) output additional memory
# profiles covering only the execution (excluding the setup/teardown) - those
# should be preferred for analysis since they more closely correspond to what's
# reported as B/op and alloc/op.
benchdiff --run=^BenchmarkSysbench/KV/3node/oltp_read_only$ --old=3944370 --new=2944efe --memprofile ./pkg/sql/tests

🔴 Sysbench [KV, 3node, oltp_write_only]

Metric	Old Commit	New Commit	Delta	Note
🔴 sec/op	3.376m ±4%	3.516m ±3%	+4.16%	p=0.005 n=15
⚪ allocs/op	4.238k ±0%	4.237k ±0%	~	p=0.544 n=15

Reproduce

benchdiff binaries:

mkdir -p benchdiff/2944efe/bin/1058449141
gcloud storage cp gs://cockroach-microbench-ci/builds/2944efe978a7b7eaa17826772f9d4ca633b6f75a/bin/pkg_sql_tests benchdiff/2944efe/bin/1058449141/cockroachdb_cockroach_pkg_sql_tests
chmod +x benchdiff/2944efe/bin/1058449141/cockroachdb_cockroach_pkg_sql_tests
mkdir -p benchdiff/3944370/bin/1058449141
gcloud storage cp gs://cockroach-microbench-ci/builds/3944370be734756fdc54b5aa04934f6c4315464c/bin/pkg_sql_tests benchdiff/3944370/bin/1058449141/cockroachdb_cockroach_pkg_sql_tests
chmod +x benchdiff/3944370/bin/1058449141/cockroachdb_cockroach_pkg_sql_tests

benchdiff command:

# NB: for best (most stable) results, also add a suitable `--benchtime` that
# results in ~1s to ~5s of benchmark runs. For example, if ops average ~3ms, a
# benchtime of `1000x` is appropriate.
#
# Some benchmarks (in particular BenchmarkSysbench) output additional memory
# profiles covering only the execution (excluding the setup/teardown) - those
# should be preferred for analysis since they more closely correspond to what's
# reported as B/op and alloc/op.
benchdiff --run=^BenchmarkSysbench/KV/3node/oltp_write_only$ --old=3944370 --new=2944efe --memprofile ./pkg/sql/tests

Artifacts

download:

mkdir -p new
gcloud storage cp gs://cockroach-microbench-ci/artifacts/2944efe978a7b7eaa17826772f9d4ca633b6f75a/23961324784-1/\* new/
mkdir -p old
gcloud storage cp gs://cockroach-microbench-ci/artifacts/3944370be734756fdc54b5aa04934f6c4315464c/23961324784-1/\* old/

built with commit: 2944efe978a7b7eaa17826772f9d4ca633b6f75a