feat: per-tier Veo video pricing

[kenobijon]

Problem

#418 merged with Veo priced as the fast variant at each (tier, audio) combination. Two refinements landed on the branch after the squash-merge and never reached main:

1. Veo manifests never expose the model variant — any variant-based floor (lite, fast, standard) is an assumption, not a detection.
2. Veo's real price rises with resolution, so its 1080p output deserves a premium over 720p.

Change

Veo videos (C2PA-blind, model_name=None) are now priced by delivered tier alone via _VEO_TIER_PRICES (audio-independent):

Tier	Price (USD/s)	Multiplier	Anchor
480p	$0.07	1.53x	= Seedance fast 480p (same resolution, same multiplier)
720p	$0.152	2.25x	= Seedance fast 720p
1080p	$0.40	3.65x	veo-3.1 standard 1080p rate

The ladder is strictly monotonic, so Veo miners have a real incentive to deliver 1080p. min(observed, requested) capping, the unknown-resolution -> baseline rule, and all Seedance/image pricing are unchanged. The OpenRouter-parsed MODEL_TIER_PRICES tables remain as the live Veo price reference and become the pricing source again if variants ever become distinguishable (manifest signal or a 4K capability probe).

Testing

Full test suite passes (37 tests). Unit-verified: per-tier prices, audio-independence, overshoot capping, unknown-resolution baseline, Seedance paths untouched.

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Note

Medium Risk
Changes how validator video rewards are computed for all C2PA-blind Veo submissions; economic incentives shift (especially 1080p vs prior fast-floor + audio), though scope is limited to rewards.py pricing logic.

Overview
Veo reward pricing for C2PA-blind videos (model_name=None) no longer assumes the veo-3.1-fast floor with per-(tier, audio) lookups from MODEL_TIER_PRICES. It now uses a dedicated _VEO_TIER_PRICES table keyed only by delivered resolution tier (480p / 720p / 1080p), audio-independent, with a monotonic ladder that pays more for 1080p.

_get_video_generation_price was simplified for the Veo branch: after effective_tier (still min(observed, requested)), it reads _VEO_TIER_PRICES instead of _VEO_FLOOR_MODEL + has_audio + Seedance fallback for sub-floor tiers. MODEL_TIER_PRICES stays in the tree as a live OpenRouter reference but is documented as not used for Veo until variants might be distinguishable again.

Version bump 4.9.4 → 4.9.5 in VERSION and gas/__init__.py. Seedance/named-model and image pricing paths are unchanged.

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