docs: update all documentation to reflect speaker endpoint implementation

- Update API-COVERAGE-ANALYSIS.md:
  - Add /speaker and /playNotification to official API table
  - Update endpoint count from 18/19 to 20/21 (95% coverage)
  - Add notification system to conclusion summary
- Update API-Endpoints-Overview.md:
  - Add comprehensive speaker endpoints documentation
  - Include TTS and URL playback examples with XML
  - Document ST-10 Series compatibility and features
- Update UNIMPLEMENTED-ENDPOINTS.md:
  - Mark speaker notification system as ✅ IMPLEMENTED
  - Update priority counts (14→12 critical, 15→13 high priority)
  - Replace implementation notes with CLI and Go client examples
- Update STATUS.md:
  - Add Phase 6: Notification System completion
  - Update endpoint count from 26→28 total endpoints
  - Add speaker notifications to production ready features
  - Document recent major updates with speaker implementation
- Update README.md:
  - Add 🔔 Smart Notifications feature to features list
  - Add speaker CLI examples and Go library usage examples
  - Add Speaker Notifications to API coverage table
  - Include SPEAKER_ENDPOINT.md in documentation links
- Update CLI-REFERENCE.md:
  - Add comprehensive speaker command section
  - Include TTS examples with multi-language support
  - Document URL content playback and beep notifications
  - Add supported languages list and compatibility notes
- Update FEATURE_HISTORY.md:
  - Add Phase 8: Speaker Notification System (February 2025)
  - Document TTS, URL playback, and beep functionality
  - Update endpoint statistics (27→29 total, 100% coverage)
  - Add speaker notification test coverage and CLI commands

All documentation now reflects the complete speaker endpoint implementation
with comprehensive examples, usage patterns, and technical details.

docs/API-Endpoints-Overview.md

@@ -332,6 +332,57 @@ Retrieves clock display settings.
 ### POST /clockDisplay ✅ **Implemented**
 Configures the clock display.
 
+### POST /speaker ✅ **Implemented**
+Plays TTS messages or URL content for notifications (ST-10 Series only).
+
+**TTS Request XML:**
+```xml
+<play_info>
+  <url>http://translate.google.com/translate_tts?ie=UTF-8&amp;tl=EN&amp;client=tw-ob&amp;q=Hello%20World</url>
+  <app_key>YOUR_APPLICATION_KEY</app_key>
+  <service>TTS Notification</service>
+  <message>Google TTS</message>
+  <reason>Hello World</reason>
+  <volume>70</volume>
+</play_info>
+```
+
+**URL Content Request XML:**
+```xml
+<play_info>
+  <url>https://example.com/audio.mp3</url>
+  <app_key>YOUR_APPLICATION_KEY</app_key>
+  <service>Music Service</service>
+  <message>Song Title</message>
+  <reason>Artist Name</reason>
+  <volume>60</volume>
+</play_info>
+```
+
+**Response XML:**
+```xml
+<status>/speaker</status>
+```
+
+**Implementation Features:**
+- Multi-language TTS support (EN, DE, ES, FR, IT, NL, PT, RU, ZH, JA, etc.)
+- Volume control with automatic restoration
+- Custom metadata for NowPlaying display
+- Pauses current content, plays notification, then resumes
+
+### GET /playNotification ✅ **Implemented**
+Plays a notification beep sound (ST-10 Series only).
+
+**Response XML:**
+```xml
+<status>/playNotification</status>
+```
+
+**Implementation:**
+- Simple double beep sound
+- Pauses current media during beep
+- Available via `PlayNotificationBeep()` method
+
 ## WebSocket Connection
 
 ### WebSocket / ✅ **Implemented**