Implement the following plan:

# Plan: Remove Balance-Based Payments, Use x402 Only

## Context
The app currently has a balance system where users fund API keys with USDC, which gets credited as a USD balance, then deducted per request. The user wants to eliminate this and use only x402 per-request wallet payments. API keys stay for agent identity, free sessions stay.

## Changes

### 1. `src/middleware/api_auth.rs` — Remove balance from ApiKeyInfo and balance check
### 2. `src/models/mod.rs` — Remove balance-related models
### 3. `src/routes/api.rs` — Remove balance deduction from generate
### 4. `src/routes/payment_helper.rs` — Same removal
### 5. `src/routes/extension_api.rs` — Rewrite transform to use x402, simplify balance
### 6. `src/routes/mod.rs` — Remove credits routes, update extension routes
### 7. `src/routes/credits.rs` — Delete entire file
### 8. `templates/credits.html` — Delete entire file
### 9. `templates/help.html` — Update references to /credits
### 10. `src/db/queries.rs` — Remove dead functions