Observability: Initial instrumentation

[i5okie]

Description

Add OTel auto-instrumentation to the didwebvh FastAPI server with centralized OTel init covering both traces and metrics, and SQLAlchemy database tracing.

Requirements

• Add opentelemetry-instrumentation-fastapi, opentelemetry-instrumentation-logging, opentelemetry-instrumentation-sqlalchemy to server/pyproject.toml
• Create server/app/otel.py for centralized OTel init — configure both TracerProvider (BatchSpanProcessor + OTLPSpanExporter) and MeterProvider (PeriodicExportingMetricReader + OTLPMetricExporter)
• Call OTel init from server/main.py before uvicorn.run (must be active before uvicorn imports the app module)
• Apply FastAPIInstrumentor.instrument_app(app) in server/app/__init__.py where app is defined (uses modern lifespan context manager)
• Apply SQLAlchemyInstrumentor().instrument(engine=self._engine) in server/app/plugins/storage.py → StorageManager.__init__, immediately after self._engine is first created

Acceptance Criteria

• FastAPI request spans visible in Tempo
• SQLAlchemy/psycopg2 query spans appear as children
• HTTP RED metrics appear in Mimir
• Log correlation enabled (trace_id present in stdout log records)
• No regression in existing test suite