Group Dependabot bumps into one PR per ecosystem; apply all pending bumps

[artoonie]

Add .github/dependabot.yml so Dependabot opens a single grouped PR per ecosystem (pip in /infra, npm in /, /static/settings, /static/datatable, and GitHub Actions) for both version and security updates, instead of one PR per package.

Apply every bump from the currently open Dependabot PRs in one go:

• Django 4.2.30 -> 5.2.16 (Bump django from 4.2.30 to 5.2.16 in /infra #633)
• baseline-browser-mapping 2.10.0 -> 2.11.22 (Bump baseline-browser-mapping from 2.10.0 to 2.11.21 #641), 2.10.28 -> 2.11.22 in /static/settings (Bump baseline-browser-mapping from 2.10.28 to 2.11.22 in /static/settings #642)
• fast-uri 3.1.5 -> 3.1.7 in / (Bump fast-uri from 3.1.5 to 3.1.7 #640) and /static/settings (Bump fast-uri from 3.1.5 to 3.1.7 in /static/settings #639)
• browserslist 4.28.1 -> 4.28.9 (Bump browserslist from 4.28.1 to 4.28.8 #638)
• @humanfs/node 0.16.6 -> 0.16.8 (Bump @humanfs/node from 0.16.6 to 0.16.8 #637)
• js-yaml 4.1.1 -> 4.3.2 (Bump js-yaml from 4.1.1 to 4.3.1 #634)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.09%. Comparing base (b293ff3) to head (7449c14).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #646      +/-   ##
==========================================
- Coverage   96.11%   96.09%   -0.02%     
==========================================
  Files         132      132              
  Lines        6358     6358              
==========================================
- Hits         6111     6110       -1     
- Misses        247      248       +1     

see 1 file with indirect coverage changes

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