fix(db): fix Postgres LIMIT=0 crash and collapse DbConfig pool fields

[bug-ops]

Summary

• SessionStore::list and 3 sibling list_* helpers bound LIMIT ? with -1 as a limit == 0 ("unlimited") sentinel — a SQLite-only convenience that PostgreSQL rejects at execution time. Added a shared zeph_db::limit_clause() helper that omits the LIMIT clause entirely when unlimited, applied consistently across all 5 affected call sites.
• Also fixes a related pre-existing bug surfaced by the new Postgres test coverage: SessionStore::get/list/get_by_conversation_id decoded created_at/updated_at as String against Postgres TIMESTAMPTZ columns, breaking every SessionStore query under the postgres feature.
• Collapsed DbConfig::max_connections/pool_size into a single pool_size field. The two fields had contradictory documentation and behavior (pool_size actually governed the Postgres pool despite being documented "SQLite only"; max_connections was dead under postgres and combined via .max() under sqlite), kept in sync only by caller convention across ~15 call sites. Pre-v1.0.0, so max_connections was removed outright rather than deprecated — confirmed via workspace-wide grep it was never a user-facing config.toml key, CLI flag, or --init wizard prompt, so no migration step is needed.

Closes #5980
Closes #5970

Test plan

• cargo +nightly fmt --check — clean
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings — clean
• cargo nextest run --config-file .github/nextest.toml --workspace --features "desktop,ide,server,chat,pdf,scheduler" --lib --bins — 12999/12999 passed
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace --features "desktop,ide,server,chat,pdf,scheduler" — clean
• RUSTFLAGS="-D warnings" cargo check --workspace --all-targets --features desktop,ide,server,chat,pdf,scheduler --locked — clean
• cargo test --doc --workspace --features "desktop,ide,server,chat,pdf,scheduler" — all pass
• Per-crate rustdoc gate for touched published crates (zeph-db, zeph-session, zeph-memory, zeph-scheduler, zeph-orchestration, zeph-mcp, zeph-durable, zeph-index), with correct feature flags per crate (postgres for zeph-db; daemon/llm-planning where modules are feature-gated) — clean
• Live-verified against real Postgres via testcontainers/Docker: zeph-session postgres_integration (3/3, including a new test covering get/get_by_conversation_id's TIMESTAMPTZ decode path), zeph-memory postgres_integration (49/49, 0 regressions)
• .local/testing/playbooks/zeph-db-dual-backend.md §14 and .local/testing/coverage-status.md updated with live-verification results
• CHANGELOG.md updated under [Unreleased]