docs: correct the S3, database migration and offline mode tutorials against the storage provider and env code

[silentoplayz]

Summary

I compared the S3 storage, database migration and offline mode tutorials with the storage provider and env code.

• S3: STORAGE_PROVIDER defaults to local, and an empty or unknown value stops startup. The value s3 is matched exactly, so S3 fails. azure is a fourth provider. Only the bucket name is strictly required, because keys fall back to the AWS credential chain, and the extra S3_* options are named. The object key is prefixed with the file id rather than suffixed, and uploads still land in the local data volume first.
• Database migration: copying webui.db alone leaves uploads and the vector store behind, so the page points to a full data-directory copy.
• Offline mode: OFFLINE_MODE=true already sets HF_HUB_OFFLINE=1 and forces the three auto-update flags off, and WHISPER_MODEL_AUTO_UPDATE=false alone does not prevent a download when the model is missing.

Related issue or discussion

None.

Checklist

• I have reviewed the relevant documentation and matched the existing style.
• This PR meets Open WebUI's contribution standards: it is accurate, relevant to users, narrowly scoped, maintainable, and not promotional content, advertising, lead generation, SEO placement, or a request to list a product, service, provider, integration, gateway, tool, or company primarily for visibility.
• I understand that PRs that do not meet these standards may be closed without review and will not be merged. Repeated, low-quality, off-topic, promotional, or intentionally misleading submissions may result in the contributor being blocked from future participation in Open WebUI repositories.

Notes for reviewers

Sources on dev: config.py (STORAGE_PROVIDER, S3_*, the *_AUTO_UPDATE expressions), storage/provider.py, routers/files.py (the id prefix), env.py (OFFLINE_MODE block, DATA_DIR), and routers/audio.py (the Whisper retry). The SQLCipher problem found by the schema audit is not addressed here. See the separate finding.