perf: cache Postgres prepared statements

[larimonious]

Summary

• cache Postgres prepared statements via deadpool-postgres prepare_cached() for query, query_one, and execute
• preserve existing Result-returning error semantics for query/prepare failures
• keep transaction and pooled-connection paths covered by the same helpers

Benchmark

Local Postgres 16 container, wrk -t4 -c32 -d4s, same temporary ntnt app/fixtures, branch vs origin/main.

Pooled query routes improved roughly 2.0–2.36x; JSON/no-DB routes stayed flat, and per-request connect(...) routes stayed bottlenecked as expected.

Representative results:

• NTNT_WORKERS=1, /db/pooled-single: 7,548.7 → 15,625.0 rps (2.07x), latency 4.19ms → 2.01ms
• NTNT_WORKERS=1, /db/pooled-twenty: 428.4 → 1,011.1 rps (2.36x), latency 73.95ms → 31.52ms
• NTNT_WORKERS=8, /db/pooled-single: 32,003.7 → 62,197.8 rps (1.94x), latency 0.99ms → 0.51ms
• NTNT_WORKERS=8, /db/pooled-twenty: 2,092.2 → 4,735.9 rps (2.26x), latency 15.26ms → 6.75ms

Verification

• cargo fmt
• cargo build --profile dev-release
• cargo test --lib -q
• cargo test --test language_features_tests --test type_checker_tests --test cli_tests
• ./target/dev-release/ntnt docs --generate
• cargo test -q
• git diff --check
• DB benchmark against origin/main and this branch

[greptile-apps]

Greptile Summary

This PR caches prepared statements for pooled Postgres calls. The main changes are:

• Adds cached helpers for query, query_one, and execute.
• Keeps explicit transaction calls on the direct query path.
• Detects stale cached-plan errors, evicts the cached statement, and retries directly.
• Adds a regression test for result-shape changes after temp table recreation.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
src/stdlib/postgres.rs	Adds cached prepared-statement helpers, transaction-safe direct paths, stale-plan eviction, and regression coverage.

_{Reviews (2): Last reviewed commit: "fix: recover stale Postgres cached plans" | Re-trigger Greptile}