Seed a default dashboard with 2 starter panels (ticket 025)

ensure_default now seeds a brand-new Default profile (not one that
already existed) with a "log_category" counter metric and an
"Overview" dashboard containing a line and a pie panel, both querying
that metric so each renders both series from a single PromQL query
like any user-created panel.

The counter classifies every line into exactly "category=error" or
"category=other" via two new fixed-label fields on metric
declarations: DeclareMetricRequest::matched_labels/unmatched_labels.
These apply literal label values instead of parsing captured text,
which is what keeps a case-insensitive match (e.g. "ERROR" vs "Error")
collapsed into one clean series instead of fragmenting by source
casing, and is also the only way to express the "line does NOT match"
side at all: plain regex alternation can't do it without lookaround,
which the `regex` crate doesn't support (a catch-all alternation
branch always wins at the search's leftmost position). Both fields
default to None and are backward compatible with existing profiles.

Seeded panels are ordinary editable/deletable dashboard state,
persisted through the normal profile save path; ensure_default never
re-seeds or duplicates them on a later call once the profile exists.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/data_value.rs

@@ -1,6 +1,7 @@
 use crate::metric_primitive::MetricPrimitive;
 use crate::timestamp::Timestamp;
 use serde::{Deserialize, Serialize};
+use std::collections::BTreeMap;
 
 /// A single parsed metric primitive extracted from a regex capture group.
 #[allow(dead_code)]
@@ -64,6 +65,10 @@ pub enum DataValue {
     MetricDeclaration {
         message_match_regex: String,
         params: Vec<RegexToMetricPrimitiveMatcher>,
+        /// See `crate::engine::DeclareMetricRequest::matched_labels` (ticket 025).
+        matched_labels: Option<BTreeMap<String, String>>,
+        /// See `crate::engine::DeclareMetricRequest::unmatched_labels` (ticket 025).
+        unmatched_labels: Option<BTreeMap<String, String>>,
     },
     /// Paired with `IndexKey::Bucket`.
     BucketValue {