feat(api): add conversation list and detail endpoints with pagination - #18

What changed

Adds read-only conversation endpoints so the dashboard can inspect past Wingman conversations. GET /api/conversations returns one summary per distinct conversation with a total count, and GET /api/conversations/{conversation_id} returns its turns in chronological order. Both support limit and offset pagination.

Part of #5
Closes #17

How it was verified

• mise run install - passed after recreating the worktree venv with the mise-pinned Python 3.13 (the default Python 3.14 cannot install onnxruntime, same note as the previous run; no lockfile change kept)
• mise run test-backend - 57 passed, including 10 new tests in backend/tests/test_conversations.py (TestListConversations 6, TestConversationDetail 4)
• mise run test-frontend - passed (the repo task currently reports no frontend tests yet)
• Offline HTTP proof against the local backend (see below). Slack auth was stubbed with the repo test pattern from backend/tests/test_logging.py, so nothing connected to a live workspace. The backend startup log showed "Starting Wingman backend..." and "Database initialized" before serving.

Proof

Backend started locally with a sqlite database seeded with two fixture conversations, Slack auth stubbed offline.

List, most recently active first:

$ curl -s http://127.0.0.1:8765/api/conversations
{"items":[{"conversation_id":"conv-b","channel_id":"C2","message_count":1,"first_message_at":"2026-01-01T09:02:00","last_message_at":"2026-01-01T09:02:00","last_message_preview":"When is the office open?"},{"conversation_id":"conv-a","channel_id":"C1","message_count":2,"first_message_at":"2026-01-01T09:00:00","last_message_at":"2026-01-01T09:01:00","last_message_preview":"Use the IT portal."}],"total":2,"limit":50,"offset":0}

Detail in chronological order:

$ curl -s http://127.0.0.1:8765/api/conversations/conv-a
{"conversation_id":"conv-a","items":[{"id":1,"conversation_id":"conv-a","user_id":"U1","channel_id":"C1","role":"user","message":"How do I reset VPN?","message_ts":null,"created_at":"2026-01-01T09:00:00"},{"id":2,"conversation_id":"conv-a","user_id":"U1","channel_id":"C1","role":"assistant","message":"Use the IT portal.","message_ts":null,"created_at":"2026-01-01T09:01:00"}],"total":2,"limit":50,"offset":0}

Missing conversation and invalid pagination:

$ curl -s http://127.0.0.1:8765/api/conversations/missing
{"detail":"Conversation not found"} HTTP:404

$ curl -s "http://127.0.0.1:8765/api/conversations?limit=0"
{"detail":[{"type":"greater_than_equal","loc":["query","limit"],"msg":"Input should be greater than or equal to 1","input":"0","ctx":{"ge":1}}]} HTTP:422

Reviewer notes

• Read-only slice. No dashboard UI, no write or delete endpoints, and no migration. The new test file is wired into mise run test-backend in mise.toml so CI gates it.
• Nothing user-facing in the frontend changed in this slice, so there is no screenshot. The capture method that succeeded was backend (a). Frontend capture (b) was not applicable.
• Full verification against a live Slack workspace is not needed for this slice, since conversations are read from the local database only.