feat(server): API relay gateway backend M0-M4

Gin + GORM + pure-Go SQLite. Users/auth (JWT), API key management with
quotas, proxy gateway with weighted channel failover and health checks,
usage/billing ledger, cross-protocol conversion (Anthropic Messages /
OpenAI Chat Completions / OpenAI Responses), and channel/model admin API.
Channels declare native API formats and auto-convert the rest.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server/internal/proxy/stream/stream.go

@@ -0,0 +1,79 @@
+package stream
+
+import (
+	"bufio"
+	"bytes"
+	"io"
+	"net/http"
+	"strings"
+)
+
+// SSEEvent is a single SSE data frame.
+type SSEEvent struct {
+	Data string // the JSON payload of the `data:` line
+	Done bool   // true when the payload is [DONE]
+}
+
+// ReadSSE reads SSE frames from r, calling fn for each `data:` line.
+// It is used both for reading upstream streams and, via a pipe, for writing
+// converted streams to the client.
+func ReadSSE(r io.Reader, fn func(SSEEvent) error) error {
+	br := bufio.NewReader(r)
+	for {
+		line, err := br.ReadString('\n')
+		if err != nil {
+			if err == io.EOF {
+				return nil
+			}
+			return err
+		}
+		line = trimCRLF(line)
+		if !bytes.HasPrefix([]byte(line), []byte("data:")) {
+			continue
+		}
+		data := line[len("data:"):]
+		data = strings.TrimPrefix(data, " ")
+		if len(data) == 0 {
+			continue
+		}
+		ev := SSEEvent{Data: data, Done: bytes.Equal([]byte(data), []byte("[DONE]"))}
+		if err := fn(ev); err != nil {
+			return err
+		}
+	}
+}
+
+// Write writes an SSE event to w and flushes it.
+func Write(w http.ResponseWriter, ev SSEEvent) error {
+	if ev.Done {
+		if _, err := io.WriteString(w, "data: [DONE]\n\n"); err != nil {
+			return err
+		}
+	} else {
+		if _, err := io.WriteString(w, "data: "+ev.Data+"\n\n"); err != nil {
+			return err
+		}
+	}
+	if f, ok := w.(http.Flusher); ok {
+		f.Flush()
+	}
+	return nil
+}
+
+// WriteRaw writes a raw SSE frame string (with trailing newlines) and flushes.
+func WriteRaw(w http.ResponseWriter, frame []byte) error {
+	if _, err := w.Write(frame); err != nil {
+		return err
+	}
+	if f, ok := w.(http.Flusher); ok {
+		f.Flush()
+	}
+	return nil
+}
+
+func trimCRLF(s string) string {
+	for len(s) > 0 && (s[len(s)-1] == '\n' || s[len(s)-1] == '\r') {
+		s = s[:len(s)-1]
+	}
+	return s
+}