Audit fixes: batch recompute, negative-baseline percentages, key-ring persistence

Three defects found reviewing the last few commits.

Deriving consumption on ingest made the batch reading endpoint quadratic. A
recompute rewrites a meter's entire consumption series, and POST
/api/v1/readings ran one per reading -- 500 readings for one meter meant 500
full rewrites. IngestByMeterAsync takes renormalize:false and the endpoint
normalizes each touched meter once after the batch.

Percentage change divided by a possibly negative baseline. A net-export meter
going from -100 to -150 exported half again as much and would have been
reported as "+50%", reading as more consumption. A non-positive baseline now
reports no basis rather than a confident lie.

The data-protection key ring had no persistent home outside Docker Compose. The
LXC installer now creates /var/lib/metervault/keys at 0700 -- the app would
otherwise create it under the default umask, leaving a key ring world-readable
-- and the Unraid template maps it, since without that every UI-entered secret
was lost whenever the container was recreated. README documents the variable
and the trust boundary: keys on disk protect against leaked database content,
not against an attacker who already has the host.

Claude-Session: https://claude.ai/code/session_01V6joyergfvVLFEizH1hJLd

deploy/unraid-template.xml

@@ -23,4 +23,6 @@
   <Config Name="API key" Target="MeterVault__ApiKeys__0" Default="" Mode="" Description="API key for the REST API (X-Api-Key header). Leave blank to leave the API open." Type="Variable" Display="always" Required="false" Mask="true"/>
 
   <Config Name="Reverse-proxy trust" Target="MeterVault__ReverseProxyTrust" Default="false" Mode="" Description="Honour X-Forwarded-User from a trusted auth proxy" Type="Variable" Display="advanced" Required="false">false</Config>
+
+  <Config Name="Secret key ring" Target="/var/lib/metervault/keys" Default="/mnt/user/appdata/metervault/keys" Mode="rw" Description="Encryption keys for connector secrets entered in the web UI. Must persist: without this mapping every stored token is lost when the container is recreated." Type="Path" Display="always" Required="true">/mnt/user/appdata/metervault/keys</Config>
 </Container>