fix(subscriptions): enforce minimum interval_seconds to prevent allowance draining (#47)

[ghzhost]

Overview

Fixes #47 by establishing a minimum interval constraint on recurring subscriptions (interval_seconds >= 60).

Context & Problem

Previously, SubscriptionService::create only checked interval_seconds <= 0 and the database migration had CHECK (interval_seconds > 0). A subscription created with interval_seconds = 1 would become due again immediately after execution, causing the keeper background loop to repeatedly execute against the payer's on-chain allowance on every single poll pass.

Solution

1. Named Minimum Constant: Defined pub const MIN_SUBSCRIPTION_INTERVAL_SECS: i64 = 60; in SubscriptionService.
2. Service Validation: SubscriptionService::create now returns AppError::Validation when req.interval_seconds < MIN_SUBSCRIPTION_INTERVAL_SECS.
3. Database Migration: Added migrations/012_min_subscription_interval.sql updating the CHECK constraint on subscriptions.interval_seconds to enforce >= 60.
4. Unit Tests: Added test asserting MIN_SUBSCRIPTION_INTERVAL_SECS bound is configured properly.

Closes #47