feat(dashboard): add API endpoints for generating and downloading screenshots

[eulloa10]

SUMMARY

1. Dashboard Screenshot Generation:

• New endpoint computes and caches screenshots based on dashboard state (permalink).

2. Dashboard Download Endpoint:

• Added endpoint for downloading dashboard screenshot as PDF or PNG.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• [ ] Has associated issue:
• [ ] Required feature flags:
• [ ] Changes UI
• [ ] Includes DB Migration (follow approval process in SIP-59)
  • [ ] Migration is atomic, supports rollback & is backwards-compatible
  • [ ] Confirm DB migration upgrade and downgrade tested
  • [ ] Runtime estimates and downtime expectations provided
• [ ] Introduces new feature or API
• [ ] Removes existing feature or API

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.07595% with 11 lines in your changes missing coverage. Please review.

Project coverage is 83.70%. Comparing base (76d897e) to head (8a0e48d). Report is 966 commits behind head on master.

Files with missing lines	Patch %	Lines
superset/tasks/thumbnails.py	15.38%	11 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #29187       +/-   ##
===========================================
+ Coverage   60.48%   83.70%   +23.22%     
===========================================
  Files        1931      518     -1413     
  Lines       76236    37722    -38514     
  Branches     8568        0     -8568     
===========================================
- Hits        46114    31577    -14537     
+ Misses      28017     6145    -21872     
+ Partials     2105        0     -2105     

Flag	Coverage Δ
hive	48.86% <45.56%> (-0.30%)	:arrow_down:
javascript	?
mysql	77.27% <86.07%> (?)
postgres	77.37% <86.07%> (?)
presto	53.45% <45.56%> (-0.35%)	:arrow_down:
python	83.70% <86.07%> (+20.22%)	:arrow_up:
sqlite	76.82% <86.07%> (?)
unit	59.15% <45.56%> (+1.53%)	:arrow_up:

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