fix: enable production PostgreSQL persistence

[franciscojavierarceo]

Summary

• make the existing persistence layer work with PostgreSQL by using portable SQL, configurable pool/timeouts, and per-conversation row locking
• preserve existing PostgreSQL data during the required INTEGER to BIGINT compatibility upgrade, while supporting read-only supervisor-managed schema validation
• fail stored requests explicitly when persistence fails, add PostgreSQL integration coverage and CI, and document production rollout and TLS guidance

This is the PostgreSQL persistence slice of #102.

Closes #103.
Refs #102.

Test Plan

• cargo fmt -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test
• uvx pre-commit run --all-files
• TEST_POSTGRES_URL=... cargo test -p agentic-server-core --lib postgres_integer_widening_upgrade_preserves_existing_state -- --ignored --nocapture
• TEST_POSTGRES_URL=... cargo test -p agentic-server-core --test postgres_storage_integration -- --ignored --nocapture
• uv run --with-requirements docs/requirements.txt mkdocs build --strict (reaches the pre-existing broken docs/design/claude-code-integration.md link to ROADMAP.md)