[core] Flatten single-callsite vector realloc functions

On ESP8266 (GCC 10.3), std::vector::push_back/emplace_back emit
separate _M_realloc_insert functions even when called from only
one site. Adding __attribute__((flatten)) inlines the realloc
path, saving the out-of-line function overhead.

Changes:
- wifi: Move set_sta_priority from header to .cpp (eliminates
  duplicate instantiation at 2 call sites) and add flatten
- web_server_base: Flatten add_handler (single push_back site)
- api: Flatten accept_new_connections_ (single emplace_back site)

Saves 160 bytes of flash on ESP8266.

esphome/components/api/api_server.cpp

@@ -195,7 +195,7 @@ void APIServer::remove_client_(size_t client_index) {
 #endif
 }
 
-void APIServer::accept_new_connections_() {
+void __attribute__((flatten)) APIServer::accept_new_connections_() {
   while (true) {
     struct sockaddr_storage source_addr;
     socklen_t addr_len = sizeof(source_addr);

esphome/components/web_server_base/web_server_base.cpp

@@ -11,7 +11,7 @@ static const char *const TAG = "web_server_base";
 
 WebServerBase *global_web_server_base = nullptr;  // NOLINT(cppcoreguidelines-avoid-non-const-global-variables)
 
-void WebServerBase::add_handler(AsyncWebHandler *handler) {
+void __attribute__((flatten)) WebServerBase::add_handler(AsyncWebHandler *handler) {
   // remove all handlers
 
 #ifdef USE_WEBSERVER_AUTH

esphome/components/wifi/wifi_component.cpp

@@ -487,6 +487,19 @@ bool WiFiComponent::matches_configured_network_(const char *ssid, const uint8_t
   return false;
 }
 
+void __attribute__((flatten)) WiFiComponent::set_sta_priority(const bssid_t bssid, int8_t priority) {
+  for (auto &it : this->sta_priorities_) {
+    if (it.bssid == bssid) {
+      it.priority = priority;
+      return;
+    }
+  }
+  this->sta_priorities_.push_back(WiFiSTAPriority{
+      .bssid = bssid,
+      .priority = priority,
+  });
+}
+
 void WiFiComponent::log_discarded_scan_result_(const char *ssid, const uint8_t *bssid, int8_t rssi, uint8_t channel) {
 #if ESPHOME_LOG_LEVEL >= ESPHOME_LOG_LEVEL_VERBOSE
   // Skip logging during roaming scans to avoid log buffer overflow

esphome/components/wifi/wifi_component.h

@@ -488,18 +488,7 @@ class WiFiComponent : public Component {
     }
     return 0;
   }
-  void set_sta_priority(const bssid_t bssid, int8_t priority) {
-    for (auto &it : this->sta_priorities_) {
-      if (it.bssid == bssid) {
-        it.priority = priority;
-        return;
-      }
-    }
-    this->sta_priorities_.push_back(WiFiSTAPriority{
-        .bssid = bssid,
-        .priority = priority,
-    });
-  }
+  void set_sta_priority(const bssid_t bssid, int8_t priority);
 
   network::IPAddresses wifi_sta_ip_addresses();
   // Remove before 2026.9.0