Generate content ideas inside the agent (conversational idea generation)

## Goal

Bring idea generation into the agent — let users generate content ideas conversationally inside the agent chat, instead of only via the standalone /api/ideas/generate flow on the Ideas page.

## Why

Idea generation currently lives in a separate page (POST /api/ideas/generate → claudeService.generateContentTopics(), Sonnet 4.5, temp 0.8). The agent is becoming the central surface (Home → agent handoff series is complete), so users should be able to ask the agent to brainstorm/generate ideas and then act on them (capture input, draft, schedule) without leaving the conversation.

## Scope / Acceptance Criteria

- [ ] Add an agent tool that generates content ideas (wrap/reuse generateContentTopics) callable from within an agent conversation.
- [ ] Generated ideas are persisted to the same topics (Ideas) table so they appear on the Ideas page and pickers (respect existing ordering rules: pending+scheduled, scheduled first).
- [ ] User can refine ideas conversationally (more like this, different angle, narrower topic) — agent re-generates with context.
- [ ] User can take an idea straight into the existing flow (capture input against it / draft a post) from the conversation.
- [ ] Respect user scoping (filter by effectiveUserId, ViewAs) and workspace isolation.
- [ ] Consider model choice — current ideas path uses Sonnet 4.5; align with agent model + AGENT_ANTHROPIC_API_KEY / getClaudeApiKey.
- [ ] Localization for any new UI strings (en/pl/fr/de).

## Notes

* Reuse claudeService.generateContentTopics() and CONTENT_TOPICS_TOOL rather than duplicating prompt logic.
* Pairs with the knowledge-base work (CAP-52): once KB is wired into idea gen, the agent idea tool should benefit automatically.
* Sibling issue: saving knowledge documents from within the agent.