docs: IPv4 renews monthly, document what happens when credit runs out

[tikinang]

What Type of Change is this?

• New Page
• Minor Fix
• Minor Improvement
• Major Improvement

Description (required)

• The dedicated IPv4 subscription renews monthly on the activation day, not every 30 days. Corrected on the pricing page, the public access reference and the public access guide.
• New "When Credit Runs Out" section on Top-up & Billing: a failed IPv4 renewal releases the address and the project falls back to shared IPv4, and a top-up does not restore it. The reference and guide link to it.

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