Gate pedigree behind a dog id set in settings

The pedigree view is now opt-in and tied to your own dog rather than an
always-present free-text search. Add the dog's SKK chip or registration
number in Settings (it rides the synced profile alongside name and
birthday); the 🌳 button stays hidden until one is set, then opens the
page and loads that dog's ancestry directly.

Make repeat opens cheap: memoise the id->hundid resolution server-side so
a cached tree is served without contacting SKK at all, and mirror the
finished tree in localStorage so the page paints instantly and shows the
last-known tree offline.

Adds config.pedigree_id (with an in-place migration for existing DBs).

README.md

@@ -96,19 +96,23 @@ events, profile and photos.
 
 ## Pedigree lookup
 
-The 🌳 page looks up a dog in **SKK Hunddata** by ISO chip number or
-registration number and renders its ancestry as a tree.
+Set your dog's SKK chip or registration number in **Settings** (it rides the
+synced profile, next to name and birthday). Once set, a 🌳 button appears that
+opens a page rendering that dog's ancestry as a tree.
 
 - SKK has no public API, so the server drives the interactive site the way a
-  browser would: it resolves the input to SKK's internal dog id, fetches the
+  browser would: it resolves the id to SKK's internal dog id, fetches the
   pedigree page (7 generations per request), and follows each generation's leaves
   deeper. A lookup returns the first generations immediately and keeps crawling in
   the background; the client polls and fills the tree in as ancestors arrive.
 - Because a deep crawl is dozens of sequential upstream requests, finished trees
-  are cached per dog in the `pedigree_cache` table (pedigrees don't change), so a
-  dog is only ever crawled once and repeat lookups are instant.
-- The lookup is behind auth like the rest of `/api/*`, and is **online-only** —
-  it needs to reach SKK.
+  are cached per dog in the `pedigree_cache` table (pedigrees don't change), and
+  the id→dog resolution is memoised, so a dog is only ever crawled once and repeat
+  opens hit SKK zero times. The client also mirrors the finished tree in
+  `localStorage`, so the page paints instantly and shows the last-known tree even
+  offline.
+- The lookup is behind auth like the rest of `/api/*`; the first trace of a new
+  dog needs to reach SKK, but after that it works from cache (including offline).
 
 ## Use it on NixOS