refactor: simplify OpenTelemetry tracing configuration

Simplifies the OpenTelemetry tracing setup by removing the redundant
OTEL_ENABLED flag and using the presence of OTEL_EXPORTER_OTLP_ENDPOINT
to determine if tracing should be enabled. This follows the standard
OpenTelemetry environment variable conventions more closely.

Changes:
- Remove OTEL_ENABLED/tracing_enabled flag in favor of checking if
  OTEL_EXPORTER_OTLP_ENDPOINT is set
- Add OTEL_EXPORTER_VERIFY_SSL configuration option for OTLP endpoints
  with self-signed certificates (defaults to false for development)
- Move HTTPXClientInstrumentor initialization to module level to ensure
  httpx calls are traced across all Nextcloud API requests
- Add tracing spans to vector sync operations (scan_user_documents)
- Fix authorization header logging to only warn about missing headers
  in OAuth mode (BasicAuth mode doesn't use Authorization headers)
- Update observability documentation to reflect simplified configuration
- Refactor Dockerfile to use --no-editable flag for uv sync

Breaking changes:
- OTEL_ENABLED environment variable is removed
- Tracing is now automatically enabled when OTEL_EXPORTER_OTLP_ENDPOINT
  is set

Migration guide:
- Remove OTEL_ENABLED=true from environment configuration
- Tracing will be enabled automatically if OTEL_EXPORTER_OTLP_ENDPOINT
  is configured

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

nextcloud_mcp_server/config.py

@@ -181,8 +181,8 @@ class Settings:
     # Observability settings
     metrics_enabled: bool = True
     metrics_port: int = 9090
-    tracing_enabled: bool = False
     otel_exporter_otlp_endpoint: Optional[str] = None
+    otel_exporter_verify_ssl: bool = False
     otel_service_name: str = "nextcloud-mcp-server"
     otel_traces_sampler: str = "always_on"
     otel_traces_sampler_arg: float = 1.0
@@ -334,8 +334,9 @@ def get_settings() -> Settings:
         # Observability settings
         metrics_enabled=os.getenv("METRICS_ENABLED", "true").lower() == "true",
         metrics_port=int(os.getenv("METRICS_PORT", "9090")),
-        tracing_enabled=os.getenv("OTEL_ENABLED", "false").lower() == "true",
         otel_exporter_otlp_endpoint=os.getenv("OTEL_EXPORTER_OTLP_ENDPOINT"),
+        otel_exporter_verify_ssl=os.getenv("OTEL_EXPORTER_VERIFY_SSL", "false").lower()
+        == "true",
         otel_service_name=os.getenv("OTEL_SERVICE_NAME", "nextcloud-mcp-server"),
         otel_traces_sampler=os.getenv("OTEL_TRACES_SAMPLER", "always_on"),
         otel_traces_sampler_arg=float(os.getenv("OTEL_TRACES_SAMPLER_ARG", "1.0")),