docs: draft database backend and SQLite delivery plan

[arvindh123]

Summary

• add a decision-complete PRD and RFC for runtime PostgreSQL/SQLite backends
• define the staged delivery roadmap: PostgreSQL-safe abstraction, full SQLite parity, and release hardening
• provide draft GitHub bodies for 1 Epic, 3 capabilities, and 16 agent-ready leaf issues
• include requirement traceability and an approval-gated publication manifest

Scope

Planning documentation only. This PR does not implement SQLite, alter source code, edit migrations, change v1 contracts, publish the planned issues, or absorb the separate serverless PostgreSQL work in #42.

Validation

• verified FR-1 through FR-10 and NFR-1 through NFR-10 are defined and traceable
• verified all 16 leaf drafts contain the required execution-contract sections
• verified dependency graph and milestone gates
• passed git diff --check
• rebased onto current origin/main

Tests were not run because this PR changes Markdown planning artifacts only.