feat: persist created_via on shortened urls

[Zingzy]

Stacked on #267 (chain: #265 -> #266 -> #267 -> this).

What

• UrlV2Doc gains a created_via field storing the validated X-Spoo-Client slug (dashboard, landing, snap, raycast, cli, bot) at creation time. Null for untagged creations.
• The header parser moves from the logging middleware to shared/client_tag.py so the middleware and the shorten route share one implementation.
• POST /api/v1/shorten parses the header and passes the slug into UrlService.create as an explicit parameter.

Why

Log-based attribution expires with log retention. Persisting provenance on the document makes "where did this link come from" queryable forever, supports per-client counts straight from Mongo, and leaves room for a created-via hint in the dashboard later.

Security and performance

The stored value is never the raw header: it must match ^[a-z0-9_-]{1,32}$ or it is dropped to null, so arbitrary header content cannot reach the database. No extra queries; one nullable field on insert. The field is not exposed on any public response DTO.

Notes

• Legacy routes are untouched; legacy clients never send the header.
• Existing documents read as null, no migration needed.

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📥 Commits

Reviewing files that changed from the base of the PR and between bc871e8 and d7207fa.

📒 Files selected for processing (7)

• middleware/logging.py
• routes/api_v1/shorten.py
• schemas/models/url.py
• services/url_service.py
• shared/client_tag.py
• tests/unit/services/test_url_service.py
• tests/unit/shared/test_client_tag.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/created-via

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[Zingzy]

Mergeable. One decision to make consciously before this lands, inline below. Everything else verified clean.

Verified:

• shorten_v1 is the only v2 creation path (the emoji router only serves the emoji set, bulk has no create), so the single plumbing point covers all tagged creations; legacy stays untouched as stated.
• created_via appears in no response DTO and no route serialization.
• Old documents and anything cached before deploy decode fine: nullable field, None default, nothing reads it yet.
• shared/client_tag.py fits the shared/ convention exactly (pure function, no I/O, no framework import), and moving the parser out of the middleware instead of importing middleware code from a route was the right direction.

Chain note: merge bottom-up and let CI re-run after each retarget; the stacked PRs only ran check_formatting because the test workflows filter on the base branch. Full unit suite passes at this tip locally (2001).