Add alert-specific URL and improve Slack notification message

[MateoLostanlen]

• Include alert ID in platform URL (e.g. /alert/38187) when available
• Shorten S3 image URL using Slack mrkdwn link format
• Fix French accents in notification text
• Fix image alt text placeholder
• Replace bare platform URL with an action-verb CTA: "Visualiser l'alerte en détail sur la plateforme Pyronear"
• Drop Slack image block — presigned/internal S3 URLs triggered invalid_blocks errors
• Make platform URL configurable via new PLATFORM_URL env var (default https://platform.pyronear.org); wired into docker-compose.yml and docker-compose.dev.yml
• Remove unused url parameter from slack_client.notify and its caller
• Log the failing payload alongside Slack error responses to ease future debugging

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.15%. Comparing base (b71cc17) to head (20e9bd5).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/app/services/slack.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #566      +/-   ##
==========================================
+ Coverage   88.07%   88.15%   +0.07%     
==========================================
  Files          51       51              
  Lines        2147     2152       +5     
==========================================
+ Hits         1891     1897       +6     
+ Misses        256      255       -1     

Flag	Coverage Δ
backend	88.17% <92.85%> (+0.07%)	⬆️
client	87.80% <ø> (ø)

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[fe51]

Hi @MateoLostanlen, thanks for the PR.

Few questions before reweing

Have you tested on a dedicated test slack channel ?

Also, could you add a screenshot of how it renders on slack ?

When I have tested in #547 it was looking like below (unfortunately the image is no more available in my example)

[fe51]

Hello @MateoLostanlen ! Thank for this PR and discussion about it ! Good to me

[fe51]

Why do you have the fallback for the platform URL in two places: in the compose file and in the config file? Shouldn't it just be one or the other?

Why do you have the fallback for the URL in two places: in the compose file and in the config file? Shouldn't it just be one or the other?

[fe51]

Thanks for the udpates, all good now !