prices: fold groceries and CS skins into one price tracker (/trackers/prices) with tabs

- tab registry in #lib/price-trackers (add a tab = route + entry + icon)
- groceries -> /trackers/prices/groceries (owner-only), CS prices -> /trackers/prices/cs (public)
- old /trackers/groceries/* and /projects/cs-prices/* redirect (301 GET, 308 POST) with query
- nav: one prices entry replaces groceries and skins; cyan price-tag icon

README.md

@@ -443,7 +443,7 @@ No public signup grants admin privileges.
 
 # CS prices
 
-`/projects/cs-prices` reads only from Postgres. Each Steam market hash name is an
+`/trackers/prices/cs` (the CS skins tab of the price tracker; the old `/projects/cs-prices` URLs redirect) reads only from Postgres. Each Steam market hash name is an
 individual listing. The separate `cs.item_dimensions` table makes it possible to
 compare or group by weapon type, weapon name, weapon plus skin, skin, wear, StatTrak, Souvenir, and
 additional imported dimensions such as collection or rarity. Group chart points