feat(settings): show + edit proxy cache TTL on Remote repo Settings tab

[knowinglyAnonymous]

Problem Statement

The proxy cache TTL for a Remote (proxy) repository is configurable over the API (PUT /api/v1/repositories/{key}/cache-ttl) and readable (GET /api/v1/repositories/{key}/cache-ttl), but the web Settings tab does not surface either. Operators currently have to use curl (or read the docs that PR #71 fixed) to inspect or change the TTL — there is no in-UI affordance.

Proposed Solution

Add a "Proxy Cache" section to the existing Settings tab (src/app/(app)/repositories/_components/repo-settings-tab.tsx), shown only when repository.repo_type === "remote":

• A read-only "Currently configured TTL" line, populated from GET /:key/cache-ttl. The endpoint already falls back to DEFAULT_CACHE_TTL_SECS (24h) when nothing is stored, so the value displayed is what the proxy will actually apply — matching the contract the doc PR chore: add Vitest coverage reporting #71 spelled out.
• An editable input for the new TTL value (in seconds, with a small helper showing the equivalent in minutes/hours/days). Backend validates the range as 1..=2_592_000 (1s to 30d); the UI mirrors that and surfaces a clear inline error for out-of-range values.
• The new field plugs into the existing form's hasChanges / "Save Changes" / "Discard" workflow already used for name / description / quota, so operators get one consistent save bar at the bottom rather than yet another submit button.
• For non-Remote repos the section is hidden entirely. Backend rejects PUT /:key/cache-ttl on non-Remote with 400 anyway (see is_cache_ttl_configurable); hiding the section avoids letting users build invalid changes that fail at save time.

API wrappers (getCacheTtl, setCacheTtl) go on repositoriesApi in src/lib/api/repositories.ts. The generated SDK already exports getCacheTtl and setCacheTtl, so this is a typed-SDK call — no hand-written apiFetch needed.

Alternatives Considered

• Standalone "Cache" tab on the repo detail page. Discoverable but heavyweight for a single integer; the Settings tab already collects all repo-level configuration so a section is a better fit.
• Inline editor on the repo header / breadcrumb area. Too noisy; cache TTL is an admin knob, not a primary navigation surface.
• Read-only display only, edits via API only. Defeats the point of the issue — we want one-click edits without leaving the UI.

Use Case

As a repository admin operating a remote (proxy) repo, when I notice upstream is publishing fresh artifacts faster than my current 24h default cache lets me serve, I want to inspect the configured TTL and shorten it (e.g. to 1h) directly from the repo's Settings tab — without dropping to curl or reading docs.

Component

Settings

Additional Context

• Backend endpoints already exist and the SDK already exposes them — this is a pure UI change.
• Companion related work: the per-artifact "Invalidate cache" button shipped via #446 / #447 covers eviction of single entries; this issue is the repo-wide TTL knob.

Pre-submission Checklist

• Searched existing issues to make sure this is not a duplicate.
• Reviewed the docs / UI to confirm the section does not already exist.