Update `docs/architecture.md` for multi-backend support

[peteski22]

Part of #257 — Phase 4 step (1).
RFC: #275.

Blocked by Phase 1 + Phase 2 complete (so docs reflect the shipped
architecture, not the planned one).

Scope

Update docs/architecture.md to reflect:

• The Store protocol layering (protocol + per-dialect concrete
  implementations + shared SQLAlchemy Core helpers).
• Multi-backend support: SQLite as default, PostgreSQL as opt-in.
• Where dialect-specific subsystems live (PRAGMAs, advisory locks,
  vector/FTS placeholders).

Definition of Done

• Docs build / preview cleanly locally.
• Lint clean.
• A short diagram (text-based) showing the protocol-to-concrete
  layering, sufficient for a new contributor to navigate the code.