feat: initial Phase 1 implementation — core framework + Docker

- Data types (SManga, SChapter, Page, MangasPage, all Filter variants)
- Source interfaces (Source, CatalogueSource) with MD5-based ID generation matching Tachiyomi/Suwayomi
- HTTP client with per-host rate limiting, cookie jar, and 429 retry
- FlareSolverr v1 client (FLARESOLVERR_URL env)
- Generic GraphQL POST helper
- goquery HTML parser wrappers
- Source registry (panics on duplicate ID)
- Multi-stage Dockerfile (golang:1.26-alpine + distroless) and compose.yml (postgres, flaresolverr, app)

internal/httpclient/graphql.go

@@ -0,0 +1,55 @@
+package httpclient
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+)
+
+type GraphQLRequest struct {
+	Query     string `json:"query"`
+	Variables any    `json:"variables,omitempty"`
+}
+
+type graphQLResponse[T any] struct {
+	Data   T `json:"data"`
+	Errors []struct {
+		Message string `json:"message"`
+	} `json:"errors"`
+}
+
+// Post sends a GraphQL request and unmarshals the `data` field into T.
+func Post[T any](ctx context.Context, client *http.Client, url string, req GraphQLRequest, headers map[string]string) (T, error) {
+	var zero T
+	body, err := json.Marshal(req)
+	if err != nil {
+		return zero, err
+	}
+
+	httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost, url, bytes.NewReader(body))
+	if err != nil {
+		return zero, err
+	}
+	httpReq.Header.Set("Content-Type", "application/json")
+	httpReq.Header.Set("Accept", "application/json")
+	for k, v := range headers {
+		httpReq.Header.Set(k, v)
+	}
+
+	resp, err := client.Do(httpReq)
+	if err != nil {
+		return zero, err
+	}
+	defer resp.Body.Close()
+
+	var gqlResp graphQLResponse[T]
+	if err := json.NewDecoder(resp.Body).Decode(&gqlResp); err != nil {
+		return zero, err
+	}
+	if len(gqlResp.Errors) > 0 {
+		return zero, fmt.Errorf("graphql: %s", gqlResp.Errors[0].Message)
+	}
+	return gqlResp.Data, nil
+}