Feature 2 — Conversion by source

Why

As the owner, I want to know which channel brings leads that actually close, so that I spend the marketing budget on referrals, the website, LinkedIn or events based on numbers instead of gut feeling.

What

• [ ] logic.conversion_by_source() returns a list with one dict per source, in the order of logic.SOURCES. Each dict has the keys source, total, won and rate.
• [ ] total is the number of leads with that source, won the number of those whose stage is won.
• [ ] rate is won / total * 100, rounded to one decimal.
• [ ] A source with no leads has total 0, won 0 and rate 0. The function does not crash on a division by zero.
• [ ] The Pipeline page has a section titled Sources showing the four rows (source, total, won, rate with a % sign).

Out of scope

• Charts.
• Filtering by date range.
• Any source outside the four in logic.SOURCES.

Done when

• pytest tests/test_feature_2.py is green, without changing the tests.
• The section shows up on the Pipeline page and the numbers match data/leads.csv.
• The work is on its own branch, with a commit message that says what you built.