fix(schema): validate metadata tags contract

[skyswordw]

Summary

• validate metadata.tags as an object with at most 50 entries
• reject non-string tag keys and values during schema v1 deserialisation
• document the validation rule and exception types in the schema v1 spec

Validation

• .venv/bin/python -m pytest tests/unit/test_schema/test_models.py tests/unit/test_storage/test_storage_providers.py -q
• .venv/bin/python -m pytest tests/unit/ -q
• .venv/bin/python -m ruff check flameiq/schema/v1/models.py tests/unit/test_schema/test_models.py
• .venv/bin/python -m ruff format --check flameiq/schema/v1/models.py tests/unit/test_schema/test_models.py
• .venv/bin/python -m mypy flameiq/schema/v1/models.py
• git diff --check

Note: make lint was not usable in this local checkout because the Makefile invokes system python3, where ruff is not installed. The equivalent .venv ruff command above passed.

[skyswordw]

Marking this ready for review.

Local checks completed before opening the PR:

• .venv/bin/python -m pytest tests/unit/test_schema/test_models.py tests/unit/test_storage/test_storage_providers.py -q
• .venv/bin/python -m pytest tests/unit/ -q
• .venv/bin/python -m ruff check flameiq/schema/v1/models.py tests/unit/test_schema/test_models.py
• .venv/bin/python -m ruff format --check flameiq/schema/v1/models.py tests/unit/test_schema/test_models.py
• .venv/bin/python -m mypy flameiq/schema/v1/models.py
• git diff --check

make lint was not usable locally because the Makefile invokes system python3, where ruff is not installed; the equivalent .venv ruff command above passed. There are no outstanding maintainer comments or review threads visible on the PR.