Add event‑driven SSE connect/disconnect triggers

[SixFive7]

Describe the Web Server enhancement

Add event‑driven automation triggers to the web_server component for the built‑in Server‑Sent Events stream (/events, used by the local Web UI):

• on_sse_client_connected
• on_sse_client_disconnected

Goals:

• Match the Native API pattern (which already has on_client_connected and on_client_disconnected triggers) so users can handle Web UI presence the same way they handle API sessions.
• Events only. No built‑in sensor or getter. Users who want a count can build it in YAML from these triggers, just as they do for API sessions.
• Event‑driven, no polling, and no private/protected access required. This removes the need for the current workaround that compiles with -Dprotected=public to read Web_Server::events_.

Optional trigger variables

• path (string) – the event source path, default "/events".
• client_info (string, optional) – a short identifier similar in spirit to the API hook. Avoid IP unless explicitly enabled by the user.

Use cases

• Consistent diagnostics: Keep a triad that behaves the same way across surfaces: API connect/disconnect events (existing) and Web UI connect/disconnect events (proposed).
• Adaptive logging: Temporarily raise log verbosity while a GUI tab is connected, then restore normal level on disconnect.
• Troubleshooting load: Correlate performance or memory pressure with the presence of active Web UI tabs.
• User‑land counters: Build a simple counter (just like with API hooks) without polling or touching protected members.
• Privacy‑first automations: React to presence without exposing IPs or detailed identifiers by default.

Current workaround

esphome:
  platformio_options:
    build_flags:
      - -Dprotected=public  # temporarily publicize protected members

web_server:
  id: web
  port: 80

globals:
  - id: web_ui_clients
    type: int
    initial_value: "0"
    restore_value: false

sensor:
  - platform: template
    id: web_ui_clients_sensor
    name: "Online GUI Clients"
    update_interval: never

interval:
  - interval: 1s
    then:
      - lambda: |-
          static int last = -1;
          int n = last;
          #ifdef USE_ARDUINO
            n = (int) id(web).events_.size();
          #endif
          #ifdef USE_ESP_IDF
            n = (int) id(web).events_.count();
          #endif
          if (n != last) {
            last = n;
            id(web_ui_clients) = n;
            id(web_ui_clients_sensor).publish_state(n);
          }

Why this is not acceptable as a long-term solution:

• Breaks encapsulation globally and may fail across updates if internals change.
• Requires polling and a global macro that can affect unrelated code.
• Not beginner-friendly and difficult to recommend.

The proposed event triggers make this hack unnecessary.

Usage examples of proposed solution

Minimal: log when a Web UI tab connects or disconnects

web_server:
  id: web
  port: 80

  on_sse_client_connected:
    then:
      - logger.log:
          level: INFO
          format: "Web UI client connected to /events"

  on_sse_client_disconnected:
    then:
      - logger.log:
          level: INFO
          format: "Web UI client disconnected from /events"

User‑land counter built from events

# Simple counter that mirrors the API-session pattern

globals:
  - id: web_ui_clients
    type: int
    initial_value: "0"
    restore_value: false

sensor:
  - platform: template
    id: web_ui_clients_sensor
    name: "Online GUI Clients"
    update_interval: never

web_server:
  id: web
  port: 80

  on_sse_client_connected:
    then:
      - lambda: |-
          id(web_ui_clients)++;
          id(web_ui_clients_sensor).publish_state(id(web_ui_clients));

  on_sse_client_disconnected:
    then:
      - lambda: |-
          if (id(web_ui_clients) > 0) id(web_ui_clients)--;
          id(web_ui_clients_sensor).publish_state(id(web_ui_clients));

Raise log level only while a GUI is open

# Start at INFO; switch to DEBUG when any GUI client is connected,
# and return to INFO when the last one disconnects.

logger:
  level: INFO
  initial_level: INFO

globals:
  - id: web_ui_clients
    type: int
    initial_value: "0"
    restore_value: false

web_server:
  id: web
  port: 80

  on_sse_client_connected:
    then:
      - lambda: |-
          id(web_ui_clients)++;
      - if:
          condition:
            lambda: "return id(web_ui_clients) == 1;"
          then:
            - logger.set_level: DEBUG
            - logger.log:
                level: INFO
                format: "GUI present, raised log level to DEBUG"

  on_sse_client_disconnected:
    then:
      - lambda: |-
          if (id(web_ui_clients) > 0) id(web_ui_clients)--;
      - if:
          condition:
            lambda: "return id(web_ui_clients) == 0;"
          then:
            - logger.set_level: INFO
            - logger.log:
                level: INFO
                format: "GUI gone, restored log level to INFO"