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/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.66.0"
+version = "0.67.0"
 edition = "2021"
 default-run = "mangalord"
 
@@ -37,6 +37,9 @@ rand = "0.8"
 sha2 = "0.10"
 subtle = "2"
 base64 = "0.22"
+# Image decode + downscale for the analysis worker (keep the page image
+# under the local vision model's token budget). Only the manga page formats.
+image = { version = "0.25", default-features = false, features = ["jpeg", "png", "webp"] }
 axum-extra = { version = "0.9", features = ["cookie", "typed-header"] }
 time = "0.3"
 infer = "0.16"
@@ -48,7 +51,7 @@ chromiumoxide = { version = "0.7", features = ["tokio-runtime", "_fetcher-rusttl
 sysinfo = { version = "0.32", default-features = false, features = ["system"] }
 nix = { version = "0.29", features = ["fs"] }
 scraper = "0.20"
-reqwest = { version = "0.12", default-features = false, features = ["rustls-tls", "socks", "cookies", "stream"] }
+reqwest = { version = "0.12", default-features = false, features = ["rustls-tls", "socks", "cookies", "stream", "json"] }
 
 [dev-dependencies]
 tempfile = "3"