✨ Add SQLite backend for local development

Summary

Add an optional SQLite backend for local development and testing without AWS or Docker dependencies.

Motivation

• Zero infrastructure for local dev
• Fast tests without moto/LocalStack
• Works offline
• Great for demos and quick prototyping

Usage

from zae_limiter import RateLimiter
from zae_limiter.backends.sqlite import SQLiteRepository

# File-based (persists across runs)
repo = SQLiteRepository(path="./rate_limits.db")
limiter = RateLimiter(repository=repo)

# In-memory (fast tests)
repo = SQLiteRepository(path=":memory:")
limiter = RateLimiter(repository=repo)

Limitations

• Single-process only (no distributed coordination)
• Not for production use
• May give false confidence about production behavior

Dependencies

• #150 - Repository Protocol extraction (must be completed first)

Acceptance Criteria

• SQLiteRepository implements RepositoryProtocol
• Supports all entity/bucket operations
• Cascade support
• Thread-safe for sync wrapper
• Optional dependency: pip install zae-limiter[sqlite]
• Clear documentation that this is for local dev only

Start with the Repository Protocol extraction in issue #150, then trace the RepositoryProtocol entry point and the existing backend operations before designing SQLiteRepository. Verify entity and bucket operations, cascade behavior, and thread-safe sync use with in-memory and file-backed SQLite. Done means the optional sqlite dependency works and the documentation clearly limits this backend to local development and testing.