feat: add message reactions and priority messages

[bertheto]

Summary

This PR adds two new first-class messaging capabilities to AgentChatBus:

1. Message Reactions (annotations) — any agent can react to a message with a short label (e.g. agree, disagree, important). Reactions are idempotent, multi-agent, and emitted as SSE events for real-time UI updates.
2. Priority Messages — messages can be classified as normal (default), urgent, or system. Priority is a dedicated column (not metadata) to allow efficient SQL filtering and a fixed default for backward compatibility.

Both features are additive and backward-compatible: existing msg_post calls without priority default to "normal", and reactions appear as a new empty [] field in msg_list responses.

Changes

Database

• New reactions table with UNIQUE constraint on (message_id, agent_id, reaction) and index on message_id
• Safe migration for messages.priority TEXT NOT NULL DEFAULT 'normal' via the existing _add_column_if_missing() helper

CRUD (src/db/crud.py)

• msg_post: new priority param with validation (normal | urgent | system)
• msg_list: new optional priority filter param
• _row_to_message: reads priority with safe fallback for legacy rows
• New functions: msg_react, msg_unreact, msg_reactions, msg_reactions_bulk
• SSE events: msg.react (only on actual insert) and msg.unreact (only on actual delete) — no spurious events for idempotent no-ops

REST API (src/main.py)

• POST /api/threads/{id}/messages: accepts priority field, returns it in response
• GET /api/threads/{id}/messages: accepts optional ?priority= filter, returns priority and reactions per message (batch-fetched)
• POST /api/messages/{id}/reactions → 201
• DELETE /api/messages/{id}/reactions/{reaction}?agent_id= → 200
• GET /api/messages/{id}/reactions → 200

MCP Tools (src/mcp_server.py, src/tools/dispatch.py)

• msg_post: new priority property in schema
• msg_list: new optional priority filter property; responses include priority and reactions
• New tools: msg_react, msg_unreact

Web UI (src/static/index.html, src/static/css/main.css)

• Priority badges: URGENT (red) and SYSTEM (blue) next to message timestamp
• Reaction pills: grouped by label with count (e.g. agree ×3)

Tests (tests/test_reactions_priority.py)

• 19 unit tests (in-memory DB, no server required)
• 10 integration tests (HTTP via test server)
• Full regression suite: all existing tests still pass

Backward Compatibility

• priority defaults to "normal" — existing callers unaffected
• reactions is a new additive field ([] when empty) — existing consumers can ignore it
• Schema migration is safe for existing databases