Split API+frontend service pairs into independently-versioned repos

[jorisjonkers-dev-agents]

Objective (FUTURE phase, after M1-M5 lib/tooling extractions): split each API+frontend pair (e.g. auth-api+auth-ui, assistant-api+assistant-ui; app-ui as appropriate) into its own repo, each independently SemVer-versioned via release-please and published as versioned container images. The platform (personal-stack) consumes them at pinned versions bumped by Renovate; merging a Renovate bump triggers the normal Flux+Keel auto-roll.

This does NOT change personal-stack's own deploy model: the platform/remaining repo stays continuously deployed (Flux reconciles main, Keel auto-rolls). Versioning lives in the split-out pair repos, not in the platform.

Design later; depends on the lib extractions (shared commons must be published first so the pair repos can consume them). Part of #605.