feat: make AI provider base URL configurable via env var

[miguelmanlyx]

What

Makes the AI provider base URL configurable via an environment variable
(OPENAI_BASE_URL) so the project can switch endpoints without code changes.

Why

Hard-coding a provider URL makes it harder to test against staging endpoints,
run local models, or route to an alternative provider during an outage.
Reading the URL from an env var is a zero-risk, backward-compatible change —
the default remains your current provider.

How

# Before
client = OpenAI(api_key=os.getenv("OPENAI_API_KEY"))

# After
client = OpenAI(
    api_key=os.getenv("OPENAI_API_KEY"),
    base_url=os.getenv("OPENAI_BASE_URL", "https://api.openai.com/v1"),
)

No existing behaviour changes when OPENAI_BASE_URL is not set.