Add sharing backend: shares CRUD + ACL enforcement everywhere (#16)

- domain: Garden gains a computed MyRole ("owner"/"editor"/"viewer"); new
  ShareWithUser (share + recipient identity); sentinels ErrShareUserNotFound,
  ErrCannotShareWithSelf, ErrShareExists.
- store/shares.go: GetShareRole, ListSharesForGarden (joined with users),
  CreateShare (UNIQUE → ErrShareExists), UpdateShareRole, DeleteShare.
- store/gardens.go: ListGardensForActor returns owned + shared-with-me gardens,
  each carrying my_role (replaces the owner-only list).
- service: requireGardenRole now consults garden_shares (owner implicit, else the
  share's role, else masked ErrNotFound) and stamps MyRole on every read-through.
  UpdateGarden tightened to OWNER-only (editors edit contents, not the garden).
  UpdatePlanting only re-checks plant visibility when the plant is CHANGED, so a
  shared editor can edit a plop that uses the owner's private plant.
- service/shares.go: ListShares/AddShare/UpdateShareRole (owner only), RemoveShare
  (owner or a recipient leaving). AddShare targets an existing account by exact
  email; unknown → 404, self → 400, duplicate → 409.
- api: GET,POST /gardens/:id/shares and PATCH,DELETE /gardens/:id/shares/:userId.

Tests: the full {owner, editor, viewer, stranger} × {read, mutate object, mutate
planting, edit garden, delete garden, manage shares} ACL matrix; shared gardens
carry my_role; AddShare error cases; role upgrade + self-leave; plus a two-user
HTTP flow. The matrix caught the editor-cannot-edit-owner's-plant bug above.

GOWORK=off go build/vet/test ./internal/... green.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01JdQpdYYsTgtkJBxbcpAszi

internal/api/shares.go

@@ -0,0 +1,88 @@
+package api
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// shareCreateRequest is the body for POST /gardens/:id/shares: the recipient's
+// exact account email and the role to grant (viewer|editor).
+type shareCreateRequest struct {
+	Email string `json:"email" binding:"required"`
+	Role  string `json:"role" binding:"required"`
+}
+
+// shareUpdateRequest is the body for PATCH /gardens/:id/shares/:userId.
+type shareUpdateRequest struct {
+	Role string `json:"role" binding:"required"`
+}
+
+func (h *handlers) listShares(c *gin.Context) {
+	gardenID, ok := parseIDParam(c, "id")
+	if !ok {
+		return
+	}
+	shares, err := h.svc.ListShares(c.Request.Context(), mustActor(c).ID, gardenID)
+	if err != nil {
+		writeServiceError(c, err)
+		return
+	}
+	c.JSON(http.StatusOK, shares)
+}
+
+func (h *handlers) addShare(c *gin.Context) {
+	gardenID, ok := parseIDParam(c, "id")
+	if !ok {
+		return
+	}
+	var req shareCreateRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		writeAPIError(c, http.StatusBadRequest, "INVALID_INPUT", "an email and a role are required")
+		return
+	}
+	sh, err := h.svc.AddShare(c.Request.Context(), mustActor(c).ID, gardenID, req.Email, req.Role)
+	if err != nil {
+		writeServiceError(c, err)
+		return
+	}
+	c.JSON(http.StatusCreated, sh)
+}
+
+func (h *handlers) updateShare(c *gin.Context) {
+	gardenID, ok := parseIDParam(c, "id")
+	if !ok {
+		return
+	}
+	userID, ok := parseIDParam(c, "userId")
+	if !ok {
+		return
+	}
+	var req shareUpdateRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		writeAPIError(c, http.StatusBadRequest, "INVALID_INPUT", "a role is required")
+		return
+	}
+	sh, err := h.svc.UpdateShareRole(c.Request.Context(), mustActor(c).ID, gardenID, userID, req.Role)
+	if err != nil {
+		writeServiceError(c, err)
+		return
+	}
+	c.JSON(http.StatusOK, sh)
+}
+
+func (h *handlers) removeShare(c *gin.Context) {
+	gardenID, ok := parseIDParam(c, "id")
+	if !ok {
+		return
+	}
+	userID, ok := parseIDParam(c, "userId")
+	if !ok {
+		return
+	}
+	if err := h.svc.RemoveShare(c.Request.Context(), mustActor(c).ID, gardenID, userID); err != nil {
+		writeServiceError(c, err)
+		return
+	}
+	c.Status(http.StatusNoContent)
+}