Add entity-aware fact resolution against Nexus (Vikunja #279)

facts gain a Subject/EntityID/ResolutionState triple and an async
enrichment worker that resolves free-text subjects to canonical Nexus
entity_ids, mirroring Praxis's enrichment-worker pattern. Ambiguous or
unreachable Nexus never guesses an entity_id — the fact stays pending
or terminal-ambiguous instead.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018ghELqYhZNLub2TXGMazqA

internal/config/config.go

@@ -84,6 +84,12 @@ type Config struct {
 	// falls back to the rule's static Base, matching pre-autotune behavior).
 	AutotuneInterval Duration `json:"autotune_interval,omitempty"`
 
+	// FactEnrichmentInterval — how often the fact-entity enrichment worker
+	// polls for facts with resolution_state='pending' and resolves their
+	// subject against Nexus. Default 30s. Only runs when Nexus is configured;
+	// no-ops (harmlessly) otherwise.
+	FactEnrichmentInterval Duration `json:"fact_enrichment_interval,omitempty"`
+
 	// Ntfy — the ntfy push sink config. nil ⇒ ntfy channel not wired.
 	// sev3 (ops soft) away + sev4 (ops hard) present + reminders away all
 	// route here; not wiring ntfy means those routes drop silently.
@@ -357,6 +363,8 @@ const (
 	DefaultRouterThreshold  = 0.55
 	DefaultQueryMinScore    = 0.55
 	DefaultToolTimeout      = 30 * time.Second
+
+	DefaultFactEnrichmentInterval = 30 * time.Second
 )
 
 // Load reads the JSON config at path and applies defaults. A missing file is
@@ -394,6 +402,9 @@ func (c *Config) applyDefaults() {
 	if c.AutotuneInterval == 0 {
 		c.AutotuneInterval = Duration(DefaultAutotuneInterval)
 	}
+	if c.FactEnrichmentInterval == 0 {
+		c.FactEnrichmentInterval = Duration(DefaultFactEnrichmentInterval)
+	}
 	// StateDir — when set, use it as the base for both db and socket if their
 	// paths are still relative (empty). If StateDir is empty, fall back to the
 	// XDG-style defaults (data dir for db, runtime dir for socket).