feat: H1 to H2 setting with the admin save conversion (tracer)

- content_h1_to_h2 and generic_anchors on the settings row (migration
  202610280001), Headings and links tab, config key
  golem15.journal.content_h1_to_h2 used when no settings row exists
- classes.ConvertH1 on the x/net/html tokenizer; the Blog Editor types
  convert while the setting is on and answer editor.h1_converted
- GET /posts/{slug} reports content_h1_to_h2 (additive key)

config/config.yaml

@@ -0,0 +1,10 @@
+# golem15.journal defaults, read as golem15.journal.<key>.
+#
+# content_h1_to_h2 is the H1 setting used when the Journal settings row
+# (Settings › Journal, Headings and links) does not exist; the row's own
+# value decides otherwise. On, every H1 saved in Journal rich text (post
+# content and category pillar text) becomes an H2 and the editor offers
+# H2 to H4. Off, H1 headings are stored as written and the editor offers
+# H1 to H4. Changing it never rewrites stored content; journal:fix-headings
+# --apply does.
+content_h1_to_h2: true