Fix clean installation and reject blank generation inputs

[Jemade]

Problem and resulting behavior

CI installation failed because setuptools discovered both app and migrations as top-level packages. A clean install then lacked the greenlet dependency required by SQLAlchemy asyncio. Generation requests also accepted blank model IDs and whitespace-only messages.

Closes #2.

Changes

• Explicitly package app and its subpackages; declare sqlalchemy[asyncio].
• Reject whitespace-only model IDs and content; trim valid model IDs while preserving message indentation and line breaks.
• Add five validation regressions, contributor instructions, issue forms and a PR template, linked from the README.

Validation

Reproduced both installation failures and baseline whitespace-only acceptance. A fresh pip install -e ".[dev]" now succeeds. 47 tests passed; Ruff lint/format checks passed; mypy reported no issues across 40 source files. Provider and webhook tests use mocks; no live paid-provider calls were made.

Review focus

Check package inclusion and async dependency declarations. Message content must remain byte-for-byte unchanged when it contains non-whitespace text.