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/auth/unified_verifier.py

@@ -231,17 +231,21 @@ class UnifiedTokenVerifier(TokenVerifier):
                 token,
                 signing_key.key,
                 algorithms=["RS256"],
-                issuer=self.settings.oidc_issuer
-                if hasattr(self.settings, "oidc_issuer")
-                else None,
+                issuer=(
+                    self.settings.oidc_issuer
+                    if hasattr(self.settings, "oidc_issuer")
+                    else None
+                ),
                 options={
                     "verify_signature": True,
                     "verify_exp": True,
                     "verify_iat": True,
-                    "verify_iss": True
-                    if hasattr(self.settings, "oidc_issuer")
-                    and self.settings.oidc_issuer
-                    else False,
+                    "verify_iss": (
+                        True
+                        if hasattr(self.settings, "oidc_issuer")
+                        and self.settings.oidc_issuer
+                        else False
+                    ),
                     "verify_aud": False,  # We handle audience validation separately
                 },
             )