feat(analysis): vision client + system prompt + AnalysisConfig

The OpenAI-compatible vision client and the bounded prompt/output handling
for the analysis worker (no DB, fully unit-tested):

- analysis::prompt: terse system prompt carrying the JSON schema, the OCR
  kind / content-warning vocabularies, and the output-size caps.
- analysis::vision: VisionClient (downscale via the new `image` dep →
  base64 data-URL → chat/completions with an image_url part), plus pure
  parse_chat_completion (fence/prose-tolerant) and sanitize (drop empty
  OCR, truncate, clamp tags to 10 via the shared page-tag normalizer,
  filter unknown warnings).
- config::AnalysisConfig extended with endpoint/model/api_key/timeouts/
  max_tokens/max_image_dim/max_image_bytes + from_env.
- Deps: add `image` (jpeg/png/webp), reqwest `json` feature. Expose
  api::page_tags::normalize_tag as pub(crate) for reuse.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/analysis/mod.rs

@@ -0,0 +1,11 @@
+//! AI content-analysis worker: calls a local OpenAI-compatible vision
+//! model on each page image and turns the result into OCR text, global
+//! auto-tags, a scene description, and NSFW content warnings.
+//!
+//! * [`prompt`] — the system prompt + the bounded output vocabulary and
+//!   sanitization helpers (pure, unit-tested).
+//! * [`vision`] — the HTTP client: downscale → request → parse → sanitize.
+//! * [`daemon`] — the job-leasing worker loop (added in the worker phase).
+
+pub mod prompt;
+pub mod vision;