Add an SQL core

[shaypal5]

No description provided.

[Copilot]

Pull Request Overview

This PR introduces a SQL-based caching core for Cachier, enabling use of SQLAlchemy backends alongside existing pickle, memory, and MongoDB options. Key changes include

• Implementation of a new _SQLCore class in src/cachier/cores/sql.py, with full CRUD and concurrency handling
• Comprehensive test suite in tests/test_sql_core.py plus CI and requirements updates to run SQL-backed tests
• Updates to cachier decorator, documentation in README.rst, and CI configurations to integrate the SQL backend

Reviewed Changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/test_sql_core.py	Add end-to-end and edge-case tests for SQL core
src/cachier/cores/sql.py	Implement the _SQLCore class using SQLAlchemy
src/cachier/core.py	Wire in backend="sql" and sql_engine parameter
tests/sql_requirements.txt	Add SQLAlchemy and psycopg2 dependencies
README.rst	Document SQL core usage, schema, and limitations
.github/workflows/ci-test.yml	Extend CI matrix to run sql-marked tests
pyproject.toml	Register sql pytest marker and lint exceptions

Comments suppressed due to low confidence (1)

tests/test_sql_core.py:64

• Consider adding a test for the next_time=True case to verify that stale entries are returned immediately when the next_time flag is enabled.

        next_time=False,

[shaypal5]

Ok, this is ready for a review.
Wanna take a look? :)
@Borda