feat: one sharing vocabulary across pastes, workspaces and the audit log

[darwin67]

Summary

The design doc's "Vocabulary" section reduces four overlapping sharing concepts to three settings. Each one reads as a sentence. This PR puts those words in the interface. Stored values are unchanged (audience, external_sharing_policy, visibility), so the API, the CLI and existing audit metadata keep working.

Setting	Was	Now
Who can see this (paste)	Workspace / Unlisted / Public	Workspace / Anyone with the link / Public
Sharing outside the team (workspace)	External sharing: Disabled / Unlisted links / Public	Members only / Members may share a link / Members may also publish
Who can join (workspace)	Visibility: Open / Private	Anyone in the organization / Invite only

Commits

1. TextbinWeb.Sharing holds every label, sentence and icon. The paste editor, detail page and list now use it. Pastes.audience_allowed?/2 is now public, so the editor disables options using the same rule the changeset enforces, not a copy of it.
2. Workspace settings show a sentence under each setting, and the sentence follows the selection before you save.
3. Workspace lists, the new-workspace form, the organization overview and the audit log all use the same labels. Older audit events render with the new words too.

⚠️ The design doc and the code disagree

The doc says tightening "Sharing outside the team" never changes pastes already shared. The code does the opposite: Organizations.clamp_paste_audiences/1 downgrades existing pastes when the setting is saved. I kept the code's behaviour, because it's the safer default for an owner shutting off external access. The wording follows the code:

• Members only: "Links shared earlier stop working."
• Members may share a link: "Public pastes become link-only."

Which one should win, the doc or the code? If the doc's behaviour is the intent, that change belongs in its own PR, together with a doc or code fix.

Test plan

• mix test: 567 tests, 0 failures. New tests:
  • Unit tests for Sharing.
  • The settings sentence follows the selection without saving.
  • Audit descriptions for join and sharing changes.
  • Paste badges and sentences for link and public pastes.
• mix credo reports no issues. mix compile --warnings-as-errors is clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_019fnjMN7WpEjQJD2zveakV6

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Generated by Claude Code

[darwin67]

Oracle review complete: signed off after correcting the invite-only workspace explanation. Existing members retain access, including those who self-joined while open; owners must add new members. No permissions, stored values, or audience-clamp behavior changed. The nearby comment now describes policy tightening on save rather than autosave. Validation: mix precommit (567 ExUnit tests plus JS tests), mix credo, and real Chromium checks of private settings and the unsaved open-policy preview passed. Both screenshots were inspected for readable wording/layout. Corrections committed and pushed without rewriting published history. Review session and visual artifacts: https://ampcode.com/threads/T-01a12594-e2f1-75be-b343-9239564f5c84

Post-push CI is green: memory/local/S3 Elixir tests, Rust tests, formatting/linting, and production-container build all passed. Release/publication jobs skipped as expected.