feat(analysis): add in-process ocrs OCR backend

Add a CPU-only, English-first OCR engine (the `ocrs` crate on the `rten`
runtime) as an alternative to the local-LLM "vision" backend, which ran
too slowly on the target Raspberry Pi 5.

- `ANALYSIS_BACKEND` (default `ocr`) selects the engine; `vision` keeps the
  existing LLM path. Deploy-time, env-only — not admin-tunable.
- `analysis::ocr` adds an `OcrEngine` trait (testable seam), the `OcrsEngine`
  impl (models loaded once, inference on the blocking pool), and an
  `OcrAnalyzeDispatcher` that plugs into the existing `AnalyzeDispatcher`
  seam and reuses `persist_analysis`, so OCR text lands in `page_ocr_text`
  and the `search_doc` tsvector exactly as the vision path produces them.
- The backend image bakes the two `.rten` models into /models, where
  `OCRS_DETECTION_MODEL` / `OCRS_RECOGNITION_MODEL` default.

Because `/v1/me/page-search` ranks on `search_doc`, OCR text search works
the moment pages are processed. Auto-tagging, scene description and NSFW
flags remain the vision backend's job (deferred).

Bumps to 0.90.0 (minor) in both manifests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/analysis/mod.rs

@@ -9,5 +9,6 @@
 
 pub mod daemon;
 pub mod events;
+pub mod ocr;
 pub mod prompt;
 pub mod vision;