feat: user profiles, subscriptions, and subscription feed

- PocketBase: new user_subscriptions collection (follower_id, followee_id)
- pocketbase.ts: subscribe/unsubscribe/getFollowingIds/getPublicProfile/
  getUserPublicLibrary/getUserCurrentlyReading/getSubscriptionFeed helpers
- GET /api/users/[username] — public profile with subscription state
- POST/DELETE /api/users/[username]/subscribe — follow/unfollow
- /users/[username] — public profile page: avatar, stats, follow button,
  currently reading grid, full library grid
- CommentsSection: usernames are now links to /users/[username]
- Home page: 'From People You Follow' section powered by subscription feed

scraper/internal/storage/pocketbase.go

@@ -449,6 +449,16 @@ func (s *PocketBaseStore) EnsureCollections(ctx context.Context) error {
 				{"name": "vote", "type": "text", "required": true}, // "up" | "down"
 			},
 		},
+		{
+			// follower_id follows followee_id
+			"name": "user_subscriptions",
+			"type": "base",
+			"fields": []map[string]interface{}{
+				{"name": "follower_id", "type": "text", "required": true},
+				{"name": "followee_id", "type": "text", "required": true},
+				{"name": "created", "type": "date"},
+			},
+		},
 	}
 	for _, col := range collections {
 		name, _ := col["name"].(string)