Add authenticated POST /api/v1/jobs to enqueue registered job kinds

Addresses feature-review finding F1 on Feature 04 PR #30: a job can now
be enqueued over the API and its progressive result read back.

- jobs: Runner.Enqueue(kind, payload) rejects an unregistered kind with
  ErrUnknownKind and a payload that does not decode into the kind's
  payload type with ErrInvalidPayload, storing no job. Kind.Enqueue
  shares the same insert path.
- server: POST /api/v1/jobs, inside the session- and CSRF-guarded /api/v1
  group, takes {kind, payload?} and returns 202 {id}; unknown kind,
  bad payload or malformed body return 400 with the standard error shape.
- Tests drive POST -> GET partial result (handler held on a channel) ->
  GET succeeded through the production router, plus 400/401/403 cases.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/jobs.go

@@ -1,7 +1,11 @@
 package server
 
 import (
+	"bytes"
+	"encoding/json"
 	"errors"
+	"fmt"
+	"io"
 	"log/slog"
 	"net/http"
 
@@ -10,6 +14,79 @@ import (
 	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/jobs"
 )
 
+// maxJobBody bounds a POST /jobs request body.
+const maxJobBody = 64 << 10
+
+// jobRequest is the POST /jobs body. Payload is optional; an omitted
+// payload is the empty object.
+type jobRequest struct {
+	Kind    string          `json:"kind"`
+	Payload json.RawMessage `json:"payload"`
+}
+
+// jobCreated is the POST /jobs response.
+type jobCreated struct {
+	ID string `json:"id"`
+}
+
+// postJob serves POST /api/v1/jobs: it enqueues a job of a registered kind
+// and returns 202 with its id, which GET /api/v1/jobs/{id} reports on.
+func postJob(logger *slog.Logger, runner *jobs.Runner) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		req, err := decodeJobRequest(http.MaxBytesReader(w, r.Body, maxJobBody))
+		if err != nil {
+			writeError(w, r, logger, http.StatusBadRequest, "invalid_request", err.Error())
+			return
+		}
+		id, err := runner.Enqueue(r.Context(), req.Kind, req.Payload)
+		switch {
+		case errors.Is(err, jobs.ErrUnknownKind):
+			writeError(w, r, logger, http.StatusBadRequest, "unknown_kind", "unknown job kind")
+			return
+		case errors.Is(err, jobs.ErrInvalidPayload):
+			writeError(w, r, logger, http.StatusBadRequest, "invalid_payload", "payload does not match the job kind")
+			return
+		case err != nil:
+			logger.ErrorContext(r.Context(), "enqueue job", "kind", req.Kind, "error", err)
+			writeError(w, r, logger, http.StatusInternalServerError, "internal", "internal error")
+			return
+		}
+		writeJSON(w, r, logger, http.StatusAccepted, jobCreated{ID: id})
+	}
+}
+
+// decodeJobRequest reads a single JSON object with a non-empty kind,
+// refusing unknown fields, a null payload and trailing data.
+func decodeJobRequest(body io.Reader) (jobRequest, error) {
+	data, err := io.ReadAll(body)
+	if err != nil {
+		var tooLarge *http.MaxBytesError
+		if errors.As(err, &tooLarge) {
+			return jobRequest{}, fmt.Errorf("request body exceeds %d bytes", tooLarge.Limit)
+		}
+		return jobRequest{}, errors.New("cannot read request body")
+	}
+	var req jobRequest
+	dec := json.NewDecoder(bytes.NewReader(data))
+	dec.DisallowUnknownFields()
+	if err := dec.Decode(&req); err != nil {
+		return jobRequest{}, errors.New("request body must be a JSON object with kind and an optional payload")
+	}
+	if _, err := dec.Token(); err != io.EOF {
+		return jobRequest{}, errors.New("request body must be a single JSON object")
+	}
+	if req.Kind == "" {
+		return jobRequest{}, errors.New("kind: required")
+	}
+	switch {
+	case req.Payload == nil:
+		req.Payload = json.RawMessage(`{}`)
+	case bytes.Equal(req.Payload, []byte("null")):
+		return jobRequest{}, errors.New("payload: must not be null")
+	}
+	return req, nil
+}
+
 // getJob serves GET /api/v1/jobs/{id}: the job's id, kind, status, latest
 // result and error.
 func getJob(logger *slog.Logger, runner *jobs.Runner) http.HandlerFunc {