Search Phase 7: Persistence — gob serialize/deserialize to disk

[hegner123]

Plan: ai/plans/api/SEARCH_PLAN.md — Phase 7

internal/search/storage.go:

• Save(path) error — serialize with magic bytes MCMS + version header, gob encode
• Load(path, cfg) (*Index, error) — deserialize, verify header, rebuild computed fields
• Rebuild(driver, cfg) (*Index, error) — full rebuild from all published content versions (both content systems), save to disk
• Persisted file is a cache — authoritative rebuild always available from DB