feat(database): add SQLite storage backend

[tao12345666333]

close #170

• Implement SQLite integration with SQLAlchemy/SQLModel
• Add support for JSON-serialized embeddings (no native vector type)
• Implement brute-force in-memory vector search for SQLite
• Add SQLite repositories for Resources, Items, Categories, and Relations
• Add documentation and integration tests

[Copilot]

Pull request overview

This pull request adds SQLite as a lightweight, file-based storage backend for MemU, providing an alternative to the existing in-memory and PostgreSQL backends. The implementation follows the existing repository pattern and includes comprehensive documentation.

Key Changes

• SQLite storage backend with SQLAlchemy/SQLModel integration using JSON-serialized embeddings
• Brute-force in-memory vector search implementation for SQLite (no native vector support)
• Complete repository implementations for Resources, Items, Categories, and Relations
• Integration tests and comprehensive documentation

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
tests/test_sqlite.py	Integration test demonstrating SQLite backend with memorize/retrieve operations
src/memu/database/sqlite/sqlite.py	Core SQLiteStore implementation managing repositories and database lifecycle
src/memu/database/sqlite/session.py	Session manager handling SQLite engine and connection lifecycle
src/memu/database/sqlite/schema.py	Schema builder with model caching and dynamic scope field support
src/memu/database/sqlite/repositories/base.py	Base repository class with shared filtering, embedding serialization utilities
src/memu/database/sqlite/repositories/resource_repo.py	Resource repository with CRUD operations and caching
src/memu/database/sqlite/repositories/memory_item_repo.py	Memory item repository with vector search and update operations
src/memu/database/sqlite/repositories/memory_category_repo.py	Category repository with get-or-create pattern
src/memu/database/sqlite/repositories/category_item_repo.py	Relation repository managing item-category links
src/memu/database/sqlite/repositories/init.py	Repository package exports
src/memu/database/sqlite/models.py	SQLModel definitions with JSON embedding properties and table configurations
src/memu/database/sqlite/init.py	Package entry point with database builder function
src/memu/database/factory.py	Updated factory to support SQLite provider selection
src/memu/database/init.py	Added sqlite to package exports
src/memu/app/settings.py	Extended configuration to support sqlite provider and updated DSN description
docs/sqlite.md	Comprehensive documentation covering setup, configuration, migration, and troubleshooting

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