feat(api,ui): showcase-workspace E3 — workspace-tagged scenario plans (pipeline tags + planner library filter)

[w7-mgfcode]

Sub-issue of #389 (umbrella: showcase workspace — preserve, restore, and replay showcase runs). Parallel after Foundation (E1 #390).

Purpose

Workspace-tagged scenario plans: showcase-created scenario_plan rows carry ["showcase", "workspace:<name>", "source:showcase"] via the existing GIN-indexed tags column, and the planner library (frontend/src/pages/visualize/planner.tsx) gains a tag filter so a workspace's plans are retrievable via GET /scenarios?tags=workspace:<name>.

Sub-tasks

To be decomposed via issue-to-subtasks when this epic is picked up.

[w7-mgfcode]

Delivered in #396 (merged to dev): pipeline-saved plans carry source:showcase + workspace:<name|workspace_id> on keep runs; planner saved-plans library gained the clickable-tag filter with ?tags= deep-link. Agent-HITL plan tagging deferred (SaveScenarioRequest has no tags field) per the PRP out-of-scope decision.