feat: Add stripe to Python deps and document dependency process

- Add stripe to Omni/Bild/Deps/Python.nix (alphabetically sorted) -
Fix all type annotations in Billing.py for mypy - Document how to add
Python packages in AGENTS.md - Add billing routes to Web.py (checkout,
portal, webhook)

This enables Stripe integration in PodcastItLater.

Related to task t-144e7lF

1 files changed, 21 insertions, 0 deletions

diff --git a/AGENTS.md b/AGENTS.md
index 5f28f2f..50e5a90 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -386,6 +386,27 @@ Run: `task init`
 - Verify dependency IDs exist: `task list`
 - Check dependency tree: `task deps <id>`
 
+## Adding New Dependencies
+
+### Python Packages
+
+To add a new Python package as a dependency:
+
+1. Add the package name to `Omni/Bild/Deps/Python.nix` (alphabetically sorted)
+2. Use it in your Python file with `# : dep <package-name>` comment at the top
+3. Run `bild <yourfile.py>` to build with the new dependency
+
+Example:
+```python
+# : out myapp
+# : dep stripe
+# : dep pytest
+import stripe
+```
+
+The package name must match the nixpkgs python package name (usually the PyPI name).
+Check available packages: `nix-env -qaP -A nixpkgs.python3Packages | grep <name>`
+
 ## Development Tools
 
 ### bild