Make subreddit list editable from the dashboard

Adds an inline subreddit editor on the dashboard that lets you add,
remove, and edit (tone, type, weight) the subreddit list, with changes
persisted to config.yaml on disk so they survive restarts.

Backend
- src/config/persistence.ts: round-trips config.yaml via
  YAML.parseDocument, mutating existing row nodes in place so their
  trailing inline `# ...` comments survive an edit; new rows are
  appended as flow-style maps to match the on-disk idiom; removed rows
  are dropped. Default values ('default' tone, 'image' type) are
  omitted on write so the file stays clean. Atomic via tmp+rename.
- src/web/routes/config.ts: GET /api/config now also returns the
  current subreddit list and the available tone keys. PUT /api/config
  accepts a `subreddits` field, validates with the existing
  SubredditConfigSchema, rejects duplicates, persists, then mutates
  ctx.config.reddit.subreddits in place so the processor picks up the
  new tone immediately without a restart.

Frontend
- public/index.html: replaces the read-only subreddit chip list in the
  Configuration snapshot section with a grid of editable rows (name,
  type, tone dropdown, weight, remove). + Add / Save buttons with
  inline status. Tones are sourced from /api/config so the catalog
  drives the dropdown.

Tests
- tests/config-persistence.test.ts: covers in-place mutation comment
  preservation, removal, appends, reorder, default omission, and
  atomic write cleanup.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/web/routes/config.ts

@@ -1,5 +1,11 @@
 import { Router } from 'express';
 import type { WebContext } from '../context.js';
+import { SubredditConfigSchema, type SubredditConfig } from '../../config/index.js';
+import { TONE_KEYS } from '../../config/tone-profiles.js';
+import { persistSubreddits } from '../../config/persistence.js';
+import { z } from 'zod';
+
+const SubredditsUpdateSchema = z.array(SubredditConfigSchema).min(1);
 
 export function createConfigRouter(ctx: WebContext): Router {
   const router = Router();
@@ -9,6 +15,8 @@ export function createConfigRouter(ctx: WebContext): Router {
       rateLimit: ctx.runtimeConfig.rateLimit,
       browser: { dryRun: ctx.runtimeConfig.browser.dryRun },
       reddit: ctx.runtimeConfig.reddit,
+      subreddits: ctx.config.reddit.subreddits,
+      tones: TONE_KEYS,
     });
   });
 
@@ -57,12 +65,36 @@ export function createConfigRouter(ctx: WebContext): Router {
       ctx.config.reddit.prioritizeRecentPosts = ctx.runtimeConfig.reddit.prioritizeRecentPosts;
       ctx.config.reddit.recentWindowMinutes = ctx.runtimeConfig.reddit.recentWindowMinutes;
 
+      if (updates.subreddits !== undefined) {
+        const parsed = SubredditsUpdateSchema.safeParse(updates.subreddits);
+        if (!parsed.success) {
+          res.status(400).json({ error: 'Invalid subreddits payload', detail: parsed.error.format() });
+          return;
+        }
+        const next: SubredditConfig[] = parsed.data;
+        const seen = new Set<string>();
+        for (const s of next) {
+          const key = s.name.toLowerCase();
+          if (seen.has(key)) {
+            res.status(400).json({ error: `Duplicate subreddit: ${s.name}` });
+            return;
+          }
+          seen.add(key);
+        }
+        persistSubreddits(next);
+        ctx.config.reddit.subreddits = next;
+      }
+
       ctx.getOnConfigChange()?.(ctx.runtimeConfig);
 
       ctx.activity.push('info', 'Config updated via dashboard');
       ctx.broadcaster.push('info', 'Configuration updated via dashboard', updates);
 
-      res.json({ success: true, config: ctx.runtimeConfig });
+      res.json({
+        success: true,
+        config: ctx.runtimeConfig,
+        subreddits: ctx.config.reddit.subreddits,
+      });
     } catch (error) {
       ctx.logger.error('Failed to update config:', error);
       res.status(400).json({ error: String(error) });