feat: conversation endpoint handlers (internal/server and cmd/conversationd)

[elevran]

Scope

Wire the ConversationStore (from #80) to HTTP handlers and mount them in cmd/conversationd. This replaces the 501 stubs created in #77 with real implementations.

Endpoints

Method	Path	Description
POST	/conversations	Create a new conversation. Body: {title?, metadata?}. Returns {id, created_at, ...}.
GET	/conversations/{id}	Return conversation metadata. 404 if not found.
DELETE	/conversations/{id}	Delete conversation and all its messages. 204 No Content.
PUT	/conversations/{id}/messages	Append one or more messages to the conversation. Body: {messages: [{role, content[]}]}. Returns {next_seq}.
GET	/conversations/{id}/messages	List messages. Query params: after_seq (uint64, default 0), limit (int, default 100).
GET	/healthz	Liveness probe (mirrors internal/server pattern).
GET	/readyz	Readiness probe (calls ConversationStore.Ping).

Handler implementation

Create internal/convserver/handlers.go (mirrors internal/server/handlers.go):

type Handler struct {
    svc convstore.ConversationStore
    log *slog.Logger
    maxBodyBytes int64
}

Error mapping follows the same pattern as internal/server:

• convstore.ErrNotFound → 404
• convstore.ErrConversationFull → 507 Insufficient Storage
• Unknown errors → 500 + log

Wire-up in cmd/conversationd

cmd/conversationd/main.go instantiates ConversationStore (Pebble or in-memory based on config), creates convserver.Handler, and serves on the configured listen address.

Unit tests

Test file: internal/convserver/handlers_test.go

Use the in-memory ConversationStore backend. Cover:

• POST /conversations — creates with and without title/metadata
• GET /conversations/{id} — found and not-found
• DELETE /conversations/{id} — idempotent
• PUT /conversations/{id}/messages — single and multi-message append; seq monotonicity
• GET /conversations/{id}/messages — pagination via after_seq and limit
• Error paths: invalid JSON body, missing required fields, store-full

Acceptance criteria

• All five endpoint handlers implemented and registered in cmd/conversationd
• /healthz and /readyz present and correct
• make presubmit passes
• Unit tests cover all endpoints including error paths
• go build ./cmd/conversationd/... succeeds with full implementation (no 501 stubs remain)

Blocked by: #80, #78