fix(config): treat an empty environment variable as unset

docker-compose.yml forwards its whitelist as "${NAME:-}", so a variable
the operator never set reaches the process as the empty string. Reads
written as a nullish fallback kept that string: the weather client built
an empty base URL and every geocode failed with ERR_INVALID_URL, and the
IP geo lookup requested a relative path.

Reads with a fallback now go through src/lib/env.ts (envValue, envOr,
envFlag), which treats empty and whitespace as unset. IP_GEO_LOOKUP_DISABLED
accepts 1, true, yes and on instead of only "1". A structural test refuses
a ?? fallback written directly against process.env anywhere in src/, and
the compose whitelist guard counts the helper's call shape as a read.

docs/self-hosting/geolite2.md

@@ -127,6 +127,9 @@ addresses they could not place. To remove that remainder too:
 IP_GEO_LOOKUP_DISABLED="1"
 ```
 
+`true` and `yes` work as well. Any other value, including an empty one, leaves
+the lookup on.
+
 The resolver then uses only the local databases and returns nothing when
 they miss, rather than asking anyone. An instance handling other people's
 health data is a reasonable place to make that trade.