Unify param declarations: metrics reference params by name (ticket 037)

Collapse the Params-panel ParamDeclaration and the metric-embedded
RegexToMetricPrimitiveMatcher into a single ParamDeclaration type
(name, regex/capture-group-or-json-field-path, kind). MetricDeclaration
now references params by name via label_params: Vec<String> and
value_param: Option<String> instead of embedding its own extractor per
param, so a param declared once in the Params panel can be reused as a
label or value by any number of metrics. Declaring a metric that
references an undeclared param name is now rejected at declare-time.

Backend:
- src/data_value.rs: unified ParamDeclaration struct; MetricDeclaration
  data value now carries label_params/value_param instead of a Vec of
  inline matchers.
- src/engine.rs: DeclareParamRequest is now an alias for
  ParamDeclaration; DeclareMetricRequest carries label_params/
  value_param; validate_metric_request checks referenced param names
  against the declared-param store; apply_metric_to_log resolves
  label_params/value_param against a per-ingest resolved-param map
  instead of reading an inline matcher list.
- src/profile.rs: MetricDeclarationRecord carries label_params/
  value_param plus a private, serialize-skipped legacy_params field
  that only ever gets populated by deserializing a pre-037 profile
  file's old "params" key. ProfileStore::load migrates any such legacy
  metric into the unified shape (reusing an existing same-shaped bare
  param by name, or creating one implicitly named
  "<metric_name>.<param_name>"), then persists the migrated store so
  the on-disk file only carries the legacy shape once. Migration is
  idempotent (deterministic naming) and covered by dedicated tests.
- src/api.rs / src/dto.rs: MetricDeclarationDto and the declare/update
  metric response bodies updated to the new shape.

Frontend:
- webapp/src/lib/api.ts: MetricDeclaration type now has label_params/
  value_param; MetricParamMatcher/ParamRole types removed.
- webapp/src/components/Metrics.tsx: the declaration form now fetches
  declared params and lets you check off label params / pick a value
  param from that list, instead of re-entering a regex/capture-group/
  kind per metric param.

Leaves ticket 038 (duration as a derived param kind) and 039 (param
query resolving declared params) untouched, as scoped.

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

webapp/src/lib/api.ts

@@ -47,22 +47,13 @@ export interface Stats {
   approx_bytes_total: number
 }
 
-export type ParamRole = 'label' | 'value'
-
-export interface MetricParamMatcher {
-  param_name: string
-  capture_group: string
-  /** See `ParamDeclaration.json_field_path`. */
-  json_field_path?: string | null
-  kind: 'string' | 'number' | 'timestamp' | 'duration'
-  /** Defaults to 'label' server-side when omitted. */
-  role?: ParamRole
-}
-
 export interface MetricDeclaration {
   name: string
   message_match_regex: string
-  params: MetricParamMatcher[]
+  /** Names of already-declared `ParamDeclaration`s (see the Params panel) whose values become this metric's bucket labels. */
+  label_params: string[]
+  /** Name of an already-declared param (if any) whose value becomes this metric's per-event bucket value instead of a label. */
+  value_param?: string | null
   /** Fixed labels applied on a match/non-match, on top of any captured params. */
   matched_labels?: Record<string, string> | null
   unmatched_labels?: Record<string, string> | null