req: add coub.com support w/ "audio + video" & "video + audio" modes

[Gesugao-san]

service name

Coub.com

service description

hey team! :3

first, huge thanks for cobalt—it's been super reliable for grabbing publicly available content from all sorts of platforms. i'd love to suggest adding support for coub.com, which is:

• completely publicly accessible (all content is available without logins)
• free (no paywalls or premium content)
• DRM-free (just good ol' web videos)

why coub needs love

1. official download option? lol no - coub doesn't provide one
2. third-party solutions? let me tell you a cautionary tale...

the rise and fall of mycoub.ru:
back in its golden era, mycoub.ru was perfect:

• clean interface
• direct "audio + video" downloads
• no nonsense

then came the dark times:

• first they removed the download buttons
• then became an ad-infested extension pusher
• their chrome extension:
  • lacks the merge modes we want
  • demands suspicious permissions
  • basically acts like spyware (tracking, ads, redirects)

now it's just a shell of its former self, and we're left without a good alternative.

what would be amazing

could cobalt add these optional download modes?

1. audio + video - full audio first, then merge with video (classic mycoub style)
2. video + audio - full video first, then overlay audio

this would be:

• 100% within cobalt's scope (public, free, non-DRM content)
• a safer alternative to shady extensions
• a return to the golden age of coub downloads

plus, coub is still super popular for:

• meme content
• music snippets
• creative looping videos

would be happy to help test or provide examples if needed! this would make the coub community so happy.

link samples

### link samples & regex
should recognize:
- regular: `https://coub.com/view/47tj88`
- shared: `https://coub.com/v/47tj88`
- mobile: `https://m.coub.com/view/47tj88`
- embed: `https://coub.com/embed/47tj88`
- stories: `https://coub.com/stories/5083967-bookmarki-ch-2` (extracts just 5083967)

optimized regex pattern:

^https?://(?:m\.)?coub\.com/(?:view|v|embed)/([a-z0-9]+)|stories/(\d+)


### key improvements
1. **clean story extraction** - only captures numeric ID after /stories/
2. **two capture groups**:
   - Group 1: video IDs (47tj88)
   - Group 2: story IDs (5083967)
3. **more efficient** - uses alternation instead of optional slug matching

### example matches:
- `https://coub.com/stories/5083967-bookmarki-ch-2` → 5083967
- `https://coub.com/view/47tj88` → 47tj88
- `https://m.coub.com/embed/abc123` → abc123

additional context

No response