feat: full OTLP collector coverage (gRPC + HTTP)

[ErickJ3]

• OTLP/HTTP now serves /v1/traces, /v1/logs and /v1/metrics with protobuf, OTLP/JSON and gzip support, plus spec-conformant responses and google.rpc.Status errors
• full data fidelity: span events/links/trace_state, log observed_time/event_name/flags/severity_text, exponential histograms, histogram buckets, summary quantiles, exemplars, resource attributes, instrumentation scope, nested (kvlist) attributes
• SQLite schema v1 migration via PRAGMA user_version, existing databases preserved
• UI renders span events (waterfall markers + detail tabs), links, resource attributes and distribution charts
• accepts SDK span/link flags and ignores unknown OTLP/JSON fields (matches protobuf wire semantics)