feat(05-02): implement config change parser for RouterOS diffs

- parse_diff_changes() extracts component, summary, raw_line from unified diffs
- RouterOS path detection converts /ip firewall filter to ip/firewall/filter
- Human-readable summaries: Added/Removed/Modified N component rules
- Fallback to system/general when no path headers found

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/services/config_change_parser.py

@@ -0,0 +1,121 @@
+"""Structured change parser for RouterOS unified diffs.
+
+Extracts component names and human-readable summaries from unified diffs
+of RouterOS configurations. Each distinct RouterOS section in the diff
+produces one change entry with component path, summary, and raw diff lines.
+"""
+
+from __future__ import annotations
+
+import re
+from collections import defaultdict
+
+
+def _path_to_component(path: str) -> str:
+    """Convert RouterOS path to component format.
+
+    '/ip firewall filter' -> 'ip/firewall/filter'
+    """
+    return path.strip().lstrip("/").replace(" ", "/")
+
+
+def _component_label(component: str) -> str:
+    """Extract human-readable label from component path.
+
+    'ip/firewall/filter' -> 'firewall filter'
+    Last two segments joined with space, or last segment if only one.
+    """
+    parts = component.split("/")
+    if len(parts) >= 2:
+        return " ".join(parts[-2:])
+    return parts[-1]
+
+
+def _make_summary(adds: int, removes: int, component: str) -> str:
+    """Generate human-readable summary for a component's changes."""
+    label = _component_label(component)
+    if adds > 0 and removes > 0:
+        n = max(adds, removes)
+        noun = "rule" if n == 1 else "rules"
+        return f"Modified {n} {label} {noun}"
+    elif adds > 0:
+        noun = "rule" if adds == 1 else "rules"
+        return f"Added {adds} {label} {noun}"
+    else:
+        noun = "rule" if removes == 1 else "rules"
+        return f"Removed {removes} {label} {noun}"
+
+
+_ROUTEROS_PATH_RE = re.compile(r"^(/[a-z][a-z0-9 /\-]*)", re.IGNORECASE)
+
+FALLBACK_COMPONENT = "system/general"
+
+
+def parse_diff_changes(diff_text: str) -> list[dict]:
+    """Parse a unified diff of RouterOS config into structured changes.
+
+    Returns list of dicts sorted by component, one per distinct component:
+        {"component": "ip/firewall/filter",
+         "summary": "Added 2 firewall filter rules",
+         "raw_line": "+add chain=..."}
+    """
+    lines = diff_text.splitlines()
+
+    current_section: str | None = None
+
+    # Track per-component: adds, removes, raw_lines
+    components: dict[str, dict] = defaultdict(
+        lambda: {"adds": 0, "removes": 0, "raw_lines": []}
+    )
+
+    for line in lines:
+        # Skip unified diff headers
+        if line.startswith("---") or line.startswith("+++") or line.startswith("@@"):
+            continue
+
+        # Determine the raw content (strip the diff prefix for path detection)
+        if line.startswith("+") or line.startswith("-"):
+            content = line[1:]
+            is_change = True
+            is_add = line.startswith("+")
+        elif line.startswith(" "):
+            content = line[1:]
+            is_change = False
+            is_add = False
+        else:
+            content = line
+            is_change = False
+            is_add = False
+
+        # Check if content contains a RouterOS path header
+        path_match = _ROUTEROS_PATH_RE.match(content.strip())
+        if path_match:
+            candidate = path_match.group(1).strip()
+            # Must have at least 2 chars after / to be a real path
+            if len(candidate) > 1:
+                current_section = _path_to_component(candidate)
+
+        # Record changed lines
+        if is_change:
+            section = current_section or FALLBACK_COMPONENT
+            components[section]["raw_lines"].append(line)
+            if is_add:
+                components[section]["adds"] += 1
+            else:
+                components[section]["removes"] += 1
+
+    # Build result list
+    result = []
+    for component in sorted(components.keys()):
+        data = components[component]
+        if data["adds"] == 0 and data["removes"] == 0:
+            continue
+        result.append(
+            {
+                "component": component,
+                "summary": _make_summary(data["adds"], data["removes"], component),
+                "raw_line": "\n".join(data["raw_lines"]),
+            }
+        )
+
+    return result