fix: enforce unique usernames (#1628)

* fix: enforce unique usernames

Registration checked for a taken name, hashed the password, then
inserted, with nothing in the schema enforcing uniqueness. Concurrent
sign-ups could create two accounts with one name, after which login,
password reset, proxy login and admin lookups by username picked
between them arbitrarily.

Add a unique index on users.username for every dialect. Existing
duplicates are renamed (later rows get a -<id> suffix, the oldest keeps
the name) rather than deleted. Registration and admin user creation now
return the same 409 when the insert loses the race.

* fix: avoid username collisions during migration

drizzle/postgres/meta/_journal.json

@@ -505,6 +505,13 @@
       "when": 1791755800419,
       "tag": "0071_drop_install_count_source",
       "breakpoints": true
+    },
+    {
+      "idx": 72,
+      "version": "7",
+      "when": 1791935282340,
+      "tag": "0072_unique_username",
+      "breakpoints": true
     }
   ]
 }