Merge duplicate items and optional AI assist #42

Problem

Popular products receive many near-identical requests ("dark mode", "night theme", "dark UI"). Votes get split, the board gets noisy, and admins have no tool to consolidate. Writing a clear title and tidying wording is also manual work.

Proposal

Merge duplicates (core, non-AI)

• Bulk action / item screen button: Merge into... (pick a target item).
• Behaviour: add source votes to target (with vote rows re-pointed so de-duplication stays correct), move comments and subscribers, record "Merged: " in the target's history, then set the source to a merged state (hidden, redirects to target, keeps an audit trail).
• Undo within a window or via the audit log.
• When submitting a new idea, show "Similar ideas" (title similarity via LIKE/FULLTEXT) and let the visitor vote on the existing one instead.

AI assist (optional, opt-in, Pro candidate)

• "Suggest duplicates" on pending submissions: compare against existing items using embeddings or an LLM prompt; show a ranked list with similarity reason; admin confirms (never auto-merges).
• "Clean up title/description": rewrite a rough submission into a clear title + short description, shown as a diff for the admin to accept.
• Auto-suggest tags.
• Bring-your-own API key stored encrypted-at-rest as an option; provider adapter interface (Claude, OpenAI, none). Send only item title/description, never emails. Clear disclosure in the readme privacy section.
• All AI calls async, with timeouts and a kill switch.

Acceptance criteria

• Merge moves votes, comments, subscribers and tags correctly; totals match before/after.
• Voters who voted on both source and target are counted once.
• Source URL redirects to target; source hidden from board and API.
• Merge is reversible/audited.
• Similar-ideas suggestion appears in the submit form and is debounced.
• AI features are off by default, require explicit key entry, and fail safely.
• No personal data is sent to the AI provider.

Dependencies

Vote de-duplication (vote rows), submissions, comments, subscribers, status history.

Priority rationale

Low. Needed at scale; implement the non-AI merge first, AI later as a Pro differentiator.