Add emoji reactions to notes during discuss

[ellgreen]

Summary

• Adds a fixed set of six emoji reactions (👍 🎉 😂 😮 👀 ❤️) that can be toggled on any note, scoped to the discuss stage only — matching when votes themselves become visible, so nothing earlier in the flow (brainstorm/group/vote) is influenced by visible reaction counts.
• Reactions broadcast live over the existing socket/broker pattern (mirrors how notes and tasks already work), with a new reactions table, DAL, model, and reaction_toggle broker handler that reuses authoriseNote to reject notes from another retro.
• Frontend adds a reaction bar + emoji picker on Note, wired up only in the discuss screen so Brainstorm/Vote are unaffected.

Test plan

• go build ./... && go vet ./... && go test ./...
• cd ui && pnpm lint && pnpm exec tsc -b && pnpm test && pnpm build
• Manually verified in the browser: created a retro, advanced it to discuss, reacted to a note (pill appeared live, correctly highlighted), toggled it off, and confirmed the state persists correctly across a reload.

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