feat(crawler): plumb TorController through FetchContext and pipelines

Adds CRAWLER_TOR_CONTROL_URL / _PASSWORD / _COOKIE_PATH /
_RECIRCUIT_MAX_ATTEMPTS to CrawlerConfig and to bin/crawler.rs's
env reads. Constructs an Option<Arc<TorController>> at daemon /
CLI startup and threads it through FetchContext,
pipeline::run_metadata_pass, and content::sync_chapter_content as
Option<&TorController>.

Pure scaffolding — the controller isn't used yet; behavior is
unchanged. Next commit wires the retry hooks and session-probe
recircuit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/crawler/source.rs

@@ -67,6 +67,10 @@ pub struct SourceChapter {
 pub struct FetchContext<'a> {
     pub browser: &'a Browser,
     pub rate: &'a crate::crawler::rate_limit::HostRateLimiters,
+    /// Optional TOR control-port client. When `Some`, retry helpers
+    /// signal `NEWNYM` between transient-page attempts so the next try
+    /// draws a fresh exit. `None` keeps pre-TOR behavior.
+    pub tor: Option<&'a crate::crawler::tor::TorController>,
 }
 
 /// Lazy iterator over discovered manga refs. The caller drives the