feat(analysis): enqueue page-analysis on upload/crawl + admin backfill endpoints

Wires the analyze_page job kind into the page-create paths and adds admin
controls, all gated on a new ANALYSIS_ENABLED flag (AppState.analysis_enabled,
config::AnalysisConfig):

- Chapter upload (api::chapters) enqueues one analyze_page job per page
  after the tx commits (best-effort; failures logged, never fatal).
- Crawler content sync (persist_pages → RETURNING id) enqueues per
  persisted page when the daemon dispatcher's analysis_enabled flag is set;
  threaded through sync_chapter_content (CLI/resync pass false).
- repo::page_analysis::enqueue_all_pages: bulk backfill via INSERT..SELECT,
  skipping done pages (only_unanalyzed) and existing pending jobs.
- New admin endpoints (RequireAdmin, 503 when disabled, audited):
  POST /admin/analysis/reenqueue and POST /admin/pages/:id/analyze (force).

Tests: upload enqueues N jobs / no-op when disabled; crawler persist_pages
enqueue gate; admin reenqueue (backfill, idempotent, only-unanalyzed, 503,
non-admin 403) and force-analyze (force flag, 404).

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

backend/src/config.rs

@@ -66,6 +66,31 @@ impl Default for UploadConfig {
     }
 }
 
+/// AI content-analysis worker configuration. Phase 2 only needs the
+/// `enabled` gate (so page-create paths know whether to enqueue analysis
+/// jobs); later phases extend this with the vision endpoint, model, and
+/// worker knobs.
+#[derive(Clone, Debug)]
+pub struct AnalysisConfig {
+    /// Master switch (`ANALYSIS_ENABLED`). When `false`, no analysis jobs
+    /// are enqueued and no worker runs. Defaults to `false`.
+    pub enabled: bool,
+}
+
+impl Default for AnalysisConfig {
+    fn default() -> Self {
+        Self { enabled: false }
+    }
+}
+
+impl AnalysisConfig {
+    pub fn from_env() -> Self {
+        Self {
+            enabled: env_bool("ANALYSIS_ENABLED", false),
+        }
+    }
+}
+
 #[derive(Clone, Debug)]
 pub struct Config {
     pub database_url: String,
@@ -89,6 +114,7 @@ pub struct Config {
     /// the check.
     pub admin_allowed_origins: Vec<String>,
     pub crawler: CrawlerConfig,
+    pub analysis: AnalysisConfig,
     /// `(username, password)` for the admin user provisioned at startup
     /// when both `ADMIN_USERNAME` and `ADMIN_PASSWORD` are set. `None`
     /// skips the bootstrap entirely. See `repo::user::bootstrap_admin`
@@ -245,6 +271,7 @@ impl Config {
                 })
                 .unwrap_or_default(),
             crawler: CrawlerConfig::from_env()?,
+            analysis: AnalysisConfig::from_env(),
             admin_bootstrap: admin_bootstrap_from_env(),
         })
     }