docs: add FTS architecture docs + fix fulltext docstring

feat: implement review items 1-7 ...

- Decompose PropertyPipeline into 8 explicit stages with PipelineContext
- Add tenacity retry (3 attempts, exponential backoff) to OllamaClient and ImageDownloader
- Add simple in-memory circuit breaker for Ollama calls
- Resize images to 1024px before base64 encoding for vision model
- Add /health endpoint (DB, Ollama, disk checks)
- Add DB performance indexes + alembic migration
- Classify AI errors: OllamaRetryableError vs OllamaFatalError
- Add strict PayloadSchema validation for ingest endpoint

Co-Authored-By: Claude <noreply@anthropic.com>