SQLite engine and schema definition

[ZFordDev]

Summary

This PR implements the SQLite engine, connection manager, and schema creation for the new database‑backed logic layer defined in Issue #42.

It introduces a lazy‑loaded SQLite connection, enforces foreign key support, and creates all required tables on first use.
This module is fully UI‑agnostic and forms the foundation for the upcoming repository API, migration system, and scheduler integration.

No wiring or integration is included in this PR.

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Type of Change

• New feature
• Workflow improvement
• UI/UX improvement
• Documentation update
• Refactor / cleanup

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Details

What this PR adds

• New module: storage/db.py
• Lazy‑loaded SQLite connection via get_connection
• Automatic schema creation on first connect
• Foreign key enforcement (PRAGMA foreign_keys = ON)
• sqlite3.Row row factory for dict‑like access
• ISO 8601 timestamp fields for all date/time columns
• Idempotent table creation for:
  • entries
  • comments
  • tags
  • entry_tags (many‑to‑many)
  • recurrence_rules
  • metadata (migration/versioning)
• init_db() helper for future startup integration

Design decisions

• Single long‑lived connection (best practice for desktop apps)
• No UI imports or dependencies
• No repository logic included yet (handled in #42C)
• No JSON migration logic included yet (handled in #42D)
• Schema is normalized and future‑proof for recurrence, tagging, and RAW mode

Side effects

• Creates the SQLite database file on first use
• Does not modify scheduler or UI behavior
• Does not read or write JSON
• No changes to existing logic

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Testing

A small temporary test harness (not included in this PR) was used to validate:

• Runs successfully
• Tested on Linux (WSL)
• Tested on Windows
• DB file is created
• Schema is created correctly
• Foreign keys are enforced
• Row factory works as expected
• No regressions found

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Related Issues

Fixes #42B
Part of #42 — Logic Layer Overhaul (JSON → SQLite)

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Additional Notes (optional)

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