M3-007: Add periodic health check scheduler to ProviderService (configurable
via PROVIDER_HEALTH_INTERVAL env, default 60s) with cached status per adapter,
and expose GET /api/providers/health endpoint returning { providers: [...] }.
M3-009: Refactor EmbeddingService to default to Ollama (nomic-embed-text, 768
dims) with OpenAI-compatible fallback when EMBEDDING_PROVIDER=openai or
OPENAI_API_KEY is set; add dimension mismatch warning for pgvector schema.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>