Health Score: cap MITRE Coverage at 100% by canonicalising tactics

STAR rules sometimes label tactics with non-canonical names (e.g. 'Stealth',
'Defense Impairment') which were counted as distinct tactics on top of the
14 canonical ATT&CK Enterprise ones, producing percentages > 100%
(observed 15/14 = 107.1% on Purple AI tenant).

Fix in get_health_score():
  - Restrict covered_tactics to the 14 canonical ATT&CK Enterprise tactics.
  - Map known STAR aliases ('Stealth', 'Defense Impairment') -> 'Defense Evasion'.
  - Derive TOTAL_TACTICS from the canonical set (single source of truth).

Result: tactics_covered = 14, mitre_pct = 100.0 (was 15 / 107.1).

backend/routers/coverage.py

@@ -1098,7 +1098,21 @@ def _compute_health(db) -> dict:
     parser_pct = round((covered_sources / total_sources * 100) if total_sources else 0.0, 1)
 
     # --- MITRE coverage ---
-    TOTAL_TACTICS = 14  # standard ATT&CK Enterprise tactic count
+    # Standard ATT&CK Enterprise tactics (14).
+    CANONICAL_TACTICS = frozenset({
+        "Reconnaissance", "Resource Development", "Initial Access", "Execution",
+        "Persistence", "Privilege Escalation", "Defense Evasion", "Credential Access",
+        "Discovery", "Lateral Movement", "Collection", "Command and Control",
+        "Exfiltration", "Impact",
+    })
+    # SentinelOne STAR rules sometimes label tactics with non-canonical names.
+    # Map them to canonical ATT&CK so we don't over-count and exceed 100%.
+    TACTIC_ALIASES = {
+        "Stealth": "Defense Evasion",
+        "Defense Impairment": "Defense Evasion",
+    }
+    TOTAL_TACTICS = len(CANONICAL_TACTICS)
+
     rules = db.query(ParsedRule).filter_by(rule_type="library").all()
     total_rules = len(rules)
     covered_tactics: set = set()
@@ -1114,7 +1128,10 @@ def _compute_health(db) -> dict:
         if tactics or techniques:
             rules_with_mitre += 1
         for t in tactics:
-            if t and t != "Uncategorized":
+            if not t or t == "Uncategorized":
+                continue
+            t = TACTIC_ALIASES.get(t, t)
+            if t in CANONICAL_TACTICS:
                 covered_tactics.add(t)
         for tech in techniques:
             k = tech.get("id") or tech.get("name")