Easy EU region selection: ROBOFLOW_REGION, auth login --region, auth set-region

[imbgar-roboflow]

Description

One switch points the SDK + CLI at the Roboflow EU data-residency platform (Linear INF-314). Three ways to set it:

roboflow auth login --region eu    # browser auth flow opens/validates against https://app.roboflow.eu/auth-cli
roboflow auth set-region eu        # persistent config setting, shown in 'roboflow auth status'
ROBOFLOW_REGION=eu ...             # env var, for CI/scripts

Sibling PR for inference-cli: roboflow/inference#2701.

How it works

• config.py gains a region registry (us default / eu) and runtime resolvers (get_effective_region(), resolve_url()). Per-URL precedence: explicit env var > explicit config-file key > region default > existing US default — so API_URL-style overrides (the staging escape hatch) keep working unchanged.
• EU map (all verified live): api/app → api/app.roboflow.eu, detection + instance-seg → serverless.roboflow.eu, dedicated deployments → eu.roboflow.cloud. Universe and semantic-seg stay on global .com — no EU instance exists.
• login() resolves the app host at call time (an import-time constant can't see a --region flag passed in the same process) and now merges the prior config on forced re-login instead of clobbering it, so region + URL overrides survive.
• auth login --api-key --region eu validates the key against the EU API; auth status (text + --json) reports region and effective URLs; the hidden root roboflow login alias forwards --region.
• Unknown region values warn on stderr and fall back to us (never raise at import).

With no region set anywhere, every URL constant is byte-identical to today — guarded by a test.

Testing

• 937 tests green (python -m unittest), including 16 new ones: back-compat guard, precedence matrix, EU mapping, login persistence/merge (incl. --force), CLI paths, invalid-region handling
• make check_code_quality (ruff + mypy) clean
• Live smoke against real EU endpoints: interactive login prints/validates against app.roboflow.eu, --api-key path validates against api.roboflow.eu, error behavior identical to US

Docs

• CLI-COMMANDS.md: region flag, set-region, ROBOFLOW_REGION, endpoint table
• README.md: "Using Roboflow EU" section

[imbgar-roboflow]

CI note: the check_code_quality failures here are unrelated to this diff — ruff 0.16.0 (released today) stabilized the CPY copyright rules, which select = ["ALL"] + unpinned CI ruff now enforce repo-wide. Main is red with the same 72 CPY001 errors. One-line fix in #514; will rebase/rerun once that merges.