[Future] Postgres migration path

[Raulgooo]

Future

Objective

Postgres migration path.

Problem

• SQLite single-node only
• Store interface partially abstracted

Fix

1. Audit SQLite-specific code
2. Complete storage.Store abstraction
3. storage/postgres.go
4. Migration tool

Files

• internal/storage/
• cmd/shark/

Acceptance Criteria

• All SQLite code abstracted
• Postgres implementation
• Migration tool
• Tests pass on both backends
• Documentation