Add CycloneDX VEX export and component_purl field to Finding

[cschneider-vertical-relevance]

Adds a component_purl field to the Finding model (with migration) so that PURL data from CycloneDX imports is stored and round-tripped. Introduces a new dojo/vex module with two authenticated API views (GET /api/v2/vex/cyclonedx/product// and .../engagement//) that serialize Dojo triage decisions as CycloneDX 1.4 VEX documents.

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Description

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If this is a new parser, the parser guide may be worth (re)reading.

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[dryrunsecurity]

This pull request includes critical sensitive edits in dojo/urls.py, dojo/models.py, and dojo/db_migrations/0265_finding_component_purl.py, which were flagged as configured codepaths requiring special review.

🔴 Configured Codepaths Edit in dojo/urls.py (drs_25dc94e4)

Vulnerability	Configured Codepaths Edit
Description	Sensitive edits detected for this file. Sensitive file paths and allowed authors can be configured in .dryrunsecurity.yaml.

🔴 Configured Codepaths Edit in dojo/models.py (drs_e7a99f14)

Vulnerability	Configured Codepaths Edit
Description	Sensitive edits detected for this file. Sensitive file paths and allowed authors can be configured in .dryrunsecurity.yaml.

🔴 Configured Codepaths Edit in dojo/db_migrations/0265_finding_component_purl.py (drs_f3d508af)

Vulnerability	Configured Codepaths Edit
Description	Sensitive edits detected for this file. Sensitive file paths and allowed authors can be configured in .dryrunsecurity.yaml.

We've notified @mtesauro.

---

Comment to provide feedback on these findings.

Report false positive: @dryrunsecurity fp [FINDING ID] [FEEDBACK]
Report low-impact: @dryrunsecurity nit [FINDING ID] [FEEDBACK]

Example: @dryrunsecurity fp drs_90eda195 This code is not user-facing

All finding details can be found in the DryRun Security Dashboard.