fix(api): add snake_case json tags to Endpoint/Task/request bodies for frontend contract

Go's encoding/json does not bridge snake_case <-> PascalCase field names,
so store.Endpoint, store.Task and the anonymous request bodies in
accounts.go/auth.go were silently decoding empty/zero values from the
frontend's snake_case JSON contract (tls_mode, role_label,
src_endpoint_id, dst_endpoint_id, src_login/pass, dst_login/pass).
Adds explicit json tags; DB layer is unaffected since pgx binds by
positional params, not struct-tag reflection.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMHQTtnQtQqL8muAXHr9kd

internal/store/endpoints.go

@@ -3,11 +3,11 @@ package store
 import "context"
 
 type Endpoint struct {
-	ID        int64
-	RoleLabel string
-	Host      string
-	Port      int
-	TLSMode   string
+	ID        int64  `json:"id"`
+	RoleLabel string `json:"role_label"`
+	Host      string `json:"host"`
+	Port      int    `json:"port"`
+	TLSMode   string `json:"tls_mode"`
 }
 
 func (s *Store) CreateEndpoint(ctx context.Context, e Endpoint) (int64, error) {