feat(repository): add SQL repository support with multiple backends

[hrntknr]

Summary

#84

Type of Change

• feat: A new feature
• fix: A bug fix
• docs: Documentation only changes
• style: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
• refactor: A code change that neither fixes a bug nor adds a feature
• perf: A code change that improves performance
• test: Adding missing tests or correcting existing tests
• build: Changes that affect the build system or external dependencies
• ci: Changes to our CI configuration files and scripts
• chore: Other changes that don't modify src or test files
• revert: Reverts a previous commit

Related Issues

[Copilot]

Pull Request Overview

This PR adds SQL repository support with multiple backends (SQLite, PostgreSQL, MySQL) as an alternative to the existing local key-value store. The implementation uses GORM as the ORM layer and provides a unified interface through the existing Repository interface.

Key Changes

• Implemented SQL repository with support for SQLite, PostgreSQL, and MySQL backends
• Added command-line flags and environment variables for configuring repository backend and DSN
• Integrated SQL repository into the main application with backend selection logic

Reviewed Changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
pkg/repository/sql.go	New SQL repository implementation with GORM-based storage for OAuth sessions, clients, and PKCE requests
pkg/repository/sql_test.go	Basic tests for SQL repository functionality covering access token sessions and unsupported driver handling
pkg/mcp-proxy/main.go	Added backend selection logic to initialize either KVS or SQL repository based on configuration
main.go	Added CLI flags and environment variables for repository backend and DSN configuration
go.mod	Added GORM and SQL driver dependencies for SQLite, PostgreSQL, and MySQL

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[codecov]

Codecov Report

❌ Patch coverage is 14.63415% with 245 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/repository/sql.go	17.21%	193 Missing and 9 partials ⚠️
pkg/mcp-proxy/main.go	0.00%	37 Missing ⚠️
main.go	0.00%	6 Missing ⚠️

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