fix video thumbnail generation for limited-range YUV videos

[claudiulataretu]

Reasoning

• Since ffmpeg 8.0 the MJPEG/JPEG encoder strictly rejects limited-range (tv) YUV input, failing with Non full-range YUV is non-standard
• Video NFTs authored with limited color range (e.g. iOS "Core Media Video" exports) therefore produced no screenshot, so extractScreenshotFromVideo returned no frame and thumbnail generation failed.

Proposed Changes

• In NftThumbnailService.extractScreenshotFromVideo, write the intermediate frame screenshots as .png instead of .jpg.
• PNG (RGB) has no full-range-YUV constraint, so frame extraction succeeds for every video format; the final JPEG thumbnail is still produced downstream by sharp in extractThumbnailFromImage, so the uploaded output is unchanged.

How to test

• Trigger thumbnail generation for a video NFT whose stream reports yuv420p(tv, ...) (limited range) and confirm a JPEG thumbnail is produced and uploaded.

[github-actions]

k6 load testing comparison.
Base Commit Hash: 64adf08
Target Commit Hash: cf34213

Metric	Base				Target				Diff
	Avg	Max	90	95	Avg	Max	90	95	Avg	Max	90	95
Accounts	48.31	1284.94	58.52	60.71	44.12	956.26	54.25	57.08	-8.68% ✅	-25.58% ✅	-7.30% ✅	-5.98% ✅
Nodes	45.94	530.45	58.23	60.49	41.73	489.48	53.93	56.36	-9.17% ✅	-7.72% ✅	-7.38% ✅	-6.82% ✅
Transactions	55.15	1258.24	59.17	62.46	52.86	1860.19	69.53	77.64	-4.14% ✅	+47.84% 🔴	+17.50% 🔴	+24.30% 🔴
Mex	45.61	294.21	58.14	60.28	41.64	345.10	53.91	56.39	-8.71% ✅	+17.30% 🔴	-7.28% ✅	-6.46% ✅
Blocks	51.68	577.54	59.09	61.66	48.58	509.95	61.12	70.79	-6.01% ✅	-11.70% ✅	+3.43% 🔴	+14.80% 🔴
Tokens	45.56	293.92	58.12	60.30	41.60	510.54	53.95	56.44	-8.69% ✅	+73.70% 🔴	-7.19% ✅	-6.40% ✅
Pool	46.05	538.52	58.21	60.46	41.40	235.72	53.92	56.40	-10.10% ✅	-56.23% ✅	-7.36% ✅	-6.72% ✅
Test Run Duration	60002.41				60001.91

Legend: Avg - Average Response Time, Max - Maximum Response Time, 90 - 90th Percentile, 95 - 95th Percentile
All times are in milliseconds.