When paste creation rate exceeds threshold, PoW difficulty
increases to slow down attackers. Decays back to base when
abuse stops.
Config:
- ANTIFLOOD_THRESHOLD: requests/window before increase (30)
- ANTIFLOOD_STEP: difficulty bits per step (2)
- ANTIFLOOD_MAX: maximum difficulty cap (28)
- ANTIFLOOD_DECAY: seconds before reducing (30)
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