chore(scripts): normalize release note footer across all backends

The Copilot backend relies on the AI to reproduce the footer text exactly,
which leads to subtle differences (URL parameter order, whitespace, emoji
encoding) compared to the git-cliff template in cliff.toml.

Fix: define CANONICAL_FOOTER as a single bash constant and pipe all three
backends (copilot, git-cliff, manual) through ensure_canonical_footer(),
which strips any existing BMAC footer block and appends the canonical one.

Also tell the AI not to generate a footer at all, since it is now added
programmatically. The manual backend gains a footer it previously lacked.

User-Impact: none

scripts/release/generate-notes

@@ -375,13 +375,8 @@ Then sections using ### (H3 headings):
 
 Skip any section that has no content.
 
-After the last section, ALWAYS end with this exact footer, no modifications:
-
----
-
-If this release saved you some money on your electricity bill, a coffee would be much appreciated! ☕
-
-[![Buy Me A Coffee](https://img.buymeacoffee.com/button-api/?text=Buy%20me%20a%20coffee&emoji=☕&slug=jpawlowski&button_colour=FFDD00&font_colour=000000&font_family=Cookie&outline_colour=000000&coffee_colour=ffffff)](https://www.buymeacoffee.com/jpawlowski)
+Do NOT add any footer or sign-off. End your response after the last content section.
+A footer is appended automatically after your response.
 
 ## TITLE SELECTION
 - Find the most user-impactful change: new sensors > bug fixes affecting data quality > reliability improvements > UI improvements > translations
@@ -466,6 +461,60 @@ End after the Buy Me A Coffee button. No meta-commentary, no explanations."
     rm -f "$TEMP_PROMPT"
 }
 
+# ============================================================================
+# Canonical footer (appended by ensure_canonical_footer, backend-independent)
+# ============================================================================
+
+# Defined here so it is the single source of truth for all backends.
+# cliff.toml also contains this footer for direct git-cliff usage;
+# ensure_canonical_footer strips any existing variant before re-appending.
+CANONICAL_FOOTER='---
+
+If this release saved you some money on your electricity bill, a coffee would be much appreciated! ☕
+
+[![Buy Me A Coffee](https://img.buymeacoffee.com/button-api/?text=Buy%20me%20a%20coffee&emoji=☕&slug=jpawlowski&button_colour=FFDD00&font_colour=000000&font_family=Cookie&outline_colour=000000&coffee_colour=ffffff)](https://www.buymeacoffee.com/jpawlowski)'
+
+# Strip any existing BMAC footer block (AI/template variant) then append the
+# canonical footer defined above. Reads from stdin, writes to stdout.
+# This guarantees all backends produce an identical footer.
+ensure_canonical_footer() {
+    awk '
+        {
+            lines[NR] = $0
+        }
+        END {
+            # Find the last buymeacoffee occurrence
+            bmac_line = 0
+            for (i = NR; i >= 1; i--) {
+                if (lines[i] ~ /buymeacoffee\.com\/jpawlowski/) {
+                    bmac_line = i
+                    break
+                }
+            }
+
+            cut_at = NR
+
+            if (bmac_line > 0) {
+                # Walk back to find the nearest --- separator before the link
+                for (i = bmac_line - 1; i >= 1; i--) {
+                    if (lines[i] ~ /^---[[:space:]]*$/) {
+                        cut_at = i - 1  # Exclude the separator itself (footer provides its own)
+                        break
+                    }
+                }
+                # No --- found: cut at the buymeacoffee line itself
+                if (cut_at == NR) cut_at = bmac_line
+            }
+
+            # Trim trailing blank lines before the cut point
+            while (cut_at > 0 && lines[cut_at] ~ /^[[:space:]]*$/) cut_at--
+
+            for (i = 1; i <= cut_at; i++) print lines[i]
+        }
+    '
+    printf "\n%s\n" "$CANONICAL_FOOTER"
+}
+
 # Backend: git-cliff (template-based)
 generate_with_gitcliff() {
     log_info "${BLUE}==> Generating with git-cliff${NC}"
@@ -829,26 +878,26 @@ if [ "$AUTO_UPDATE_AVAILABLE" = "true" ]; then
     TEMP_NOTES=$(mktemp)
     case "$BACKEND" in
     copilot)
-        generate_with_copilot >"$TEMP_NOTES"
+        generate_with_copilot | ensure_canonical_footer >"$TEMP_NOTES"
         ;;
     git-cliff)
-        generate_with_gitcliff >"$TEMP_NOTES"
+        generate_with_gitcliff | ensure_canonical_footer >"$TEMP_NOTES"
         ;;
     manual)
-        generate_with_manual >"$TEMP_NOTES"
+        generate_with_manual | ensure_canonical_footer >"$TEMP_NOTES"
         ;;
     esac
 else
     # No auto-update, just output to stdout
     case "$BACKEND" in
     copilot)
-        generate_with_copilot
+        generate_with_copilot | ensure_canonical_footer
         ;;
     git-cliff)
-        generate_with_gitcliff
+        generate_with_gitcliff | ensure_canonical_footer
         ;;
     manual)
-        generate_with_manual
+        generate_with_manual | ensure_canonical_footer
         ;;
     esac