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>

docs/observability.md

@@ -16,8 +16,7 @@ The Nextcloud MCP Server includes comprehensive observability features for produ
 export METRICS_ENABLED=true
 export METRICS_PORT=9090
 
-# Enable tracing (optional)
-export OTEL_ENABLED=true
+# Enable tracing (optional - tracing is enabled when OTEL_EXPORTER_OTLP_ENDPOINT is set)
 export OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4317
 
 # Start the server
@@ -46,8 +45,7 @@ helm install nextcloud-mcp charts/nextcloud-mcp-server \
 |----------|---------|-------------|
 | `METRICS_ENABLED` | `true` | Enable Prometheus metrics |
 | `METRICS_PORT` | `9090` | Port for metrics endpoint |
-| `OTEL_ENABLED` | `false` | Enable OpenTelemetry tracing |
-| `OTEL_EXPORTER_OTLP_ENDPOINT` | - | OTLP gRPC endpoint (e.g., `http://otel-collector:4317`) |
+| `OTEL_EXPORTER_OTLP_ENDPOINT` | - | OTLP gRPC endpoint (e.g., `http://otel-collector:4317`). Tracing is enabled when this is set. |
 | `OTEL_SERVICE_NAME` | `nextcloud-mcp-server` | Service name in traces |
 | `OTEL_TRACES_SAMPLER` | `always_on` | Trace sampling strategy |
 | `OTEL_TRACES_SAMPLER_ARG` | `1.0` | Sampling rate (0.0-1.0) |