feat(errors): disambiguate plan quota exhausted from zero credit balance in image_generate error

[stackbilt-admin]

Problem

When an authenticated Stackbilder subscriber exhausts their monthly plan quota and has no credit balance, they receive the same `insufficient_credits` error as an unsubscribed user with a zero balance:

```json
{
"error": {
"code": -32602,
"message": "Insufficient credits for image_generate. Purchase a credit pack at https://stackbilder.com/img-forge#credits to continue.",
"data": { "reason": "insufficient_credits", "purchase_url": "..." }
}
}
```

This is misleading. A Teams or Pro subscriber who has used their monthly allocation should know:

• Their subscription quota is exhausted (not that they have "no credits")
• When the quota resets
• That buying a credit pack is an option, but upgrading isn't the right call

Desired behaviour

Distinguish the two cases in the error response:

Case A — Plan quota exhausted, zero credits:
```json
{
"error": {
"code": -32602,
"message": "Monthly image quota exhausted for your plan. Buy a credit pack to continue now, or wait for quota reset on [date].",
"data": {
"reason": "quota_exhausted",
"quota_resets_at": "",
"purchase_url": "https://stackbilder.com/img-forge#credits"
}
}
}
```

Case B — No plan quota AND zero credits (free tier fully consumed, or unsubscribed):
```json
{
"error": {
"code": -32602,
"message": "Insufficient credits for image_generate. Purchase a credit pack at https://stackbilder.com/img-forge#credits to continue.",
"data": {
"reason": "insufficient_credits",
"purchase_url": "https://stackbilder.com/img-forge#credits"
}
}
}
```

Context

• allow_promotion_codes: true is already live on all checkout sessions
• The gateway already has reserveQuota → consumeCredits two-phase flow; the quota reservation response should carry enough context to distinguish the cases
• Edge-auth's reserveQuota / consumeCredits will need to return the reason code + reset timestamp; see companion issue in edge-auth

Related

• edge-auth: quota reason code + reset timestamp in reservation response (filed separately)
• wiki: agent-mcp-credit-packs — Error Reference table needs a quota_exhausted row

[stackbilt-admin]

Shipped in PR #65, deployed to mcp.stackbilt.dev as Version 06685470. quota_exhausted and insufficient_credits are now distinct error paths with reset date and purchase URL respectively.