Add unit tests for analytics_db and pincopy

[vijaxx]

Adds pytest coverage for two pure/self-contained pieces:

• tests/test_analytics_db.py -- schema creation is idempotent, and the UNIQUE(theme, idx) constraint on pins actually rejects duplicates.
• tests/test_pincopy.py -- extract_json correctly pulls a JSON array out of surrounding prose (or returns None on garbage), and the template fallback() always returns 5 variants with titles under Pinterest's 100-char limit.

Also wires pytest into CI via a new requirements-dev.txt and a test step in the workflow.