feat(examples): add bot-bigbuckbunny video reply example

[ernado]

Summary

Adds a new examples/bot-bigbuckbunny example: a bot that replies to any incoming message with a video (Big Buck Bunny by default, overridable via -url).

It demonstrates:

• Multi-connection upload pool — client.Pool(N) fed to the uploader, with uploader.WithThreads(N) so part uploads fan out across the sub-connections. -pool=false falls back to the main connection for comparison.
• Progress logging — a custom uploader.Progress that logs at INFO throttled to every 5s, including instantaneous and average upload speed.
• TTY-aware colored summary to stdout after each upload (size, MIME, resolution, upload time, avg speed, transport, thumbnail).
• Local caching — the video is downloaded to the OS temp dir once (atomic rename on success) and reused.

Optional ffmpeg integration (via github.com/ernado/ff)

Best-effort; skipped with a warning when ffmpeg/ffprobe are not in PATH:

• Probes width/height/duration (set with Resolution/Duration).
• Derives the MIME type from the container format.
• Generates and attaches a JPEG thumbnail.
• -cut <dur> trims the video (ffmpeg -t; lossless stream copy when not also compressing).
• -max-size <MB> re-encodes to approximately fit a size budget (target bitrate derived from size and the possibly-cut duration; skipped when the source already fits).

Notes

• New external deps (ernado/ff + transitive OTel/sdk) land only in the examples module's go.mod/go.sum, not the core td module.
• -max-size is single-pass -b:v targeting, so output size is approximate.

Test plan

• go build ./bot-bigbuckbunny/, go vet, and gofmt -l all clean.
• Manual: set BOT_TOKEN/APP_ID/APP_HASH, run, message the bot; verify video reply, progress logs, and summary.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.24%. Comparing base (5e4cd68) to head (68ae386).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1778   +/-   ##
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  Coverage   71.24%   71.24%           
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  Files         503      503           
  Lines       23593    23593           
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  Hits        16809    16809           
  Misses       5551     5551           
  Partials     1233     1233           

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