feat: add billing surface for Hosted Switchboard

[GautamKumarOffical]

Summary

Adds billing surface for Hosted Switchboard with Standard/Pro tier support.

Closes #87

What this PR does

Implements the foundational billing system for Hosted Switchboard customers:

Billing Tiers

• Standard: 1000 tx/month included, $0.001/tx overage
• Pro: 10000 tx/month included, $0.0008/tx overage (10% annual discount)

Features

• BillingTier enum for tier selection
• TierConfig with included tx, overage rates, pricing
• BillingManager for customer metering
• Per-tx counter and overage calculation
• Monthly invoice generation

Files

• switchboard/billing.py — Billing module
• tests/test_billing.py — Unit tests

Acceptance Criteria

• Tier definitions (Standard, Pro)
• Per-tx metering
• Overage calculation
• Invoice generation
• Stripe integration (follow-up PR)
• Customer dashboard (follow-up PR)

[abhicris]

Welcome to kcolbchain, @GautamKumarOffical — glad you're here. 🌱

Here's what happens from this PR:

1. Our automated review looks for obvious issues (tests, secrets, size) within a couple of hours.
2. If it's clean and CI passes, we merge without back-and-forth.
3. If we need changes, we'll leave a specific comment — not a generic nit. Push another commit and we re-review.

While you wait:

• Run the repo's tests locally (see the repo README.md).
• Keep the PR scoped to one concern — bigger PRs land slower.
• Don't commit tokens or .env contents.

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Thanks for writing the code. We're building this to last.

[abhicris]

🤖 Audit verdict: needs_human

Code lacks monthly reset mechanism for transaction counts, causing incorrect billing calculations across months.

Risks flagged:

• Monthly billing cycle not enforced: tx_count_current_month in CustomerBilling is never reset, so transactions accumulate across months
• get_tx_count() docstring promises 'current month's tx count' but performs no month-based filtering despite recording timestamps
• Tests do not cover multi-month scenarios; single-month test cases don't catch the reset bug

Audited by the kcolbchain PR pipeline. See pipeline docs.

[gtx20060124-bot]

Good catch @abhicris. Suggestion: add a method gated by a cron-like check on — if the stored timestamp is > 30 days old, reset to 0 and update the timestamp. This keeps it pull-based without needing an external scheduler.