fix(feedback): align word-filter severity vocabulary with the admin UI

WordFilterManager.tsx sends low/moderate/high/block; the validator only
accepted mild/moderate/severe, so 3 of the 4 UI levels 400'd with "Invalid
severity level" -- including "block", the strongest advertised tier.

Aligning isIn() alone would have made "block" accepted but semantically
inert: feedbackModeration.js branches on 'severe'/'moderate', so "block"
would fall through to the flag-only branch and behave as the weakest level.
Map the UI vocabulary onto the existing outcomes instead, per the legend the
UI itself renders: block -> reject, moderate/high -> needs approval,
low -> flag only.

'severe' stays an accepted alias in the blocking predicate so any row written
through the old validator (the field is optional, so a direct API caller
could have stored one) keeps blocking. No data migration needed: the column
is a bare varchar(20) default 'moderate' with no CHECK, no enum and no seed
rows, and 'mild' already lands in the flag-only branch that 'low' now means.

Refs testplan REPORT.md #2 (Part 3, J.11).

backend/src/services/feedbackModeration.js

@@ -66,17 +66,20 @@ class FeedbackModerationService {
         }
       }
       
-      // Check for severe violations
-      if (violations.some(v => v.severity === 'severe')) {
+      // Check for blocking violations. 'severe' is the legacy vocabulary the
+      // validator used to accept before it was aligned with the UI's
+      // low/moderate/high/block levels — rows stored under it still apply.
+      const isBlocking = (v) => v.severity === 'block' || v.severity === 'severe';
+      if (violations.some(isBlocking)) {
         return {
           approved: false,
           reason: 'Content contains prohibited words',
-          violations: violations.filter(v => v.severity === 'severe')
+          violations: violations.filter(isBlocking)
         };
       }
-      
-      // Check for moderate violations
-      if (violations.some(v => v.severity === 'moderate')) {
+
+      // Check for moderate/high violations
+      if (violations.some(v => v.severity === 'moderate' || v.severity === 'high')) {
         return {
           approved: false,
           reason: 'Content requires moderation',
@@ -84,7 +87,7 @@ class FeedbackModerationService {
         };
       }
       
-      // Check for mild violations (may just flag for review)
+      // Check for low-severity violations (may just flag for review)
       if (violations.length > 0) {
         return {
           approved: true,