fix: escape % characters in DATABASE for Alembic config

[ESnark]

🐛 Bug-fix PR

Before opening this PR please:

1. make lint - passes ruff, mypy, pylint
2. make test - all unit + integration tests green
3. make coverage - ≥ 90 %
4. make docker docker-run-ssl or make podman podman-run-ssl
5. Update relevant documentation.
6. Tested with sqlite and postgres + redis.
7. Manual regression no longer fails. Ensure the UI and /version work correctly.

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📌 Summary

Bootstrap script fails if DATABASE_URL contains % characters. #1533

🔁 Reproduction Steps

1. Set DATABASE_URL with a password containing %.
2. Run the bootstrap script or start the application:
   python3 -m mcpgateway.bootstrap_db or python3 -m mcpgateway.

🐞 Root Cause

Alembic's Config.set_main_option() uses Python's configparser, which interprets % as interpolation syntax and raises InterpolationSyntaxError for URL-encoded characters like %40.

💡 Fix Description

Escape % to %% before passing the URL to configparser.

🧪 Verification

Check	Command	Status
Lint suite	make lint
Unit tests	make test
Coverage ≥ 90 %	make coverage
Manual regression no longer fails	steps / screenshots

📐 MCP Compliance (if relevant)

• Matches current MCP spec
• No breaking change to MCP clients

✅ Checklist

• Code formatted (make black isort pre-commit)
• No secrets/credentials committed