Add Publix API client for savings and store location endpoints

HTTP client that wraps the Publix services API with two endpoints:
- /api/v4/savings — fetches weekly ad deals for a given store number
- /api/v1/storelocation — finds nearby stores by ZIP code

Includes request types (SavingsResponse, SavingItem, StoreResponse,
Store) mapping directly to the Publix JSON schema. The client sends
a PublixStore header for store-scoped requests and uses a 15-second
timeout. Tests use httptest servers to verify header propagation,
JSON decoding, and error handling for non-200 responses.

internal/api/types.go

@@ -0,0 +1,41 @@
+package api
+
+// SavingsResponse is the top-level response from the Publix savings API.
+type SavingsResponse struct {
+	Savings                       []SavingItem `json:"Savings"`
+	WeeklyAdLatestUpdatedDateTime string       `json:"WeeklyAdLatestUpdatedDateTime"`
+	IsPersonalizationEnabled      bool         `json:"IsPersonalizationEnabled"`
+	LanguageID                    int          `json:"LanguageId"`
+}
+
+// SavingItem represents a single deal/saving from the weekly ad.
+type SavingItem struct {
+	ID                 string   `json:"id"`
+	Title              *string  `json:"title"`
+	Description        *string  `json:"description"`
+	Savings            *string  `json:"savings"`
+	Department         *string  `json:"department"`
+	Brand              *string  `json:"brand"`
+	Categories         []string `json:"categories"`
+	AdditionalDealInfo *string  `json:"additionalDealInfo"`
+	ImageURL           *string  `json:"imageUrl"`
+	StartFormatted     string   `json:"wa_startDateFormatted"`
+	EndFormatted       string   `json:"wa_endDateFormatted"`
+}
+
+// StoreResponse is the top-level response from the store locator API.
+type StoreResponse struct {
+	Stores []Store `json:"Stores"`
+}
+
+// Store represents a Publix store location.
+type Store struct {
+	Key      string `json:"KEY"`
+	Name     string `json:"NAME"`
+	Addr     string `json:"ADDR"`
+	City     string `json:"CITY"`
+	State    string `json:"STATE"`
+	Zip      string `json:"ZIP"`
+	Distance string `json:"DISTANCE"`
+	Phone    string `json:"PHONE"`
+}