Enhance KiCad API with stock quantity, datasheet URL, and HTTP caching

[Sebbeben]

Summary

Adds three improvements to the KiCad HTTP Library API ():

• Stock quantity field — Exposes total in-stock quantity as a \ field and storage locations as . Excludes expired lots and lots with unknown quantity. Engineers can now see parts availability directly in KiCad's symbol chooser.
• Datasheet URL resolution — The \ field now resolves to an actual datasheet file URL instead of always using the Part-DB page link. Searches attachments by type name containing "datasheet", then by attachment name, then falls back to the first PDF attachment, and finally to the Part-DB page URL.
• HTTP cache headers — Adds \ and \ headers to all API responses. Supports conditional requests () returning \ when data hasn't changed. Categories/part lists cache for 5 minutes, part details for 1 minute.

Also fixes a pre-existing bug in \ which was actually testing Part 1 instead of Part 2.

Related issues

• Addresses discussion Part-DB inventory as KiCAD library? #395 (datasheet URL passthrough)
• Partially addresses Select Fields which appear in KiCad #536 (configurable field exposure — supplier part numbers are now exposed)
• Stock quantity was a top community request for the KiCad integration

New fields exposed

Field	Visibility	Description
\	Hidden	Total quantity across all valid (non-expired, known) lots
\	Hidden	Comma-separated list of storage locations with stock
\	Hidden	Direct link to the part's info page in Part-DB

Test plan

• Added 14 unit tests in \ covering stock calculation (zero, single, multiple lots, expired/unknown exclusion), datasheet resolution (type name, attachment name, PDF fallback, space variant), storage locations, and field visibility
• Added HTTP cache header tests and 304 conditional request test in - [x] Fixed \ to test actual Part 2 with EDA inheritance from category/footprint
• Manually tested all three features on a live Part-DB instance with KiCad 9

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.86%. Comparing base (c2a51e5) to head (02869f2).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Services/EDA/KiCadHelper.php	90.90%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1232      +/-   ##
============================================
+ Coverage     54.72%   54.86%   +0.14%     
- Complexity     7918     7938      +20     
============================================
  Files           596      596              
  Lines         25468    25516      +48     
============================================
+ Hits          13937    13999      +62     
+ Misses        11531    11517      -14     

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[jbtronics]

From some quick look:

• I thing the behavior what gets exposed as datasheet URL should be configurable. A system setting that switches between yours and the old behavior would be good, as I think some people value being able to jump to Part-DB directly (and at least in my experiments the datasheet field was the only one which gets rendered as link)

[Sebbeben]

Thanks for the feedback! I've added a configurable system setting "Datasheet field links to PDF" (Settings → KiCad/EDA) that switches between the new PDF URL behavior and the old Part-DB page link. It defaults to enabled (PDF) but users can toggle it. Also configurable via EDA_KICAD_DATASHEET_AS_PDF env var.

I've opened a more comprehensive PR #1241 that includes this fix along with several other KiCad improvements (API v2 with volatile fields, batch EDA editing, configurable field export, BOM multi-footprint merging, etc.). Closing this PR in favor of #1241.