feat(prepare-db): add helper functions for generic plan collection, with optional HypoPG index; wrap each prepare-db step in begin/commit block; introduce "postgres_ai" schema

• Add postgres_ai schema for organizing our objects
• Move pg_statistic view from public to postgres_ai schema
• Add postgres_ai.explain_generic() function (SECURITY DEFINER) for collecting generic query plans with optional HypoPG index testing
• Update search_path to include postgres_ai first
• Change transaction model to wrap each step in its own begin/commit instead of grouping non-optional steps in a single transaction
• Update verification to check for postgres_ai schema and new function

Relates to: #68 (closed)