feat: add RSS feed endpoint and configuration

Implements /feed.xml route with caching and ETag support.

Features:
- GET /feed.xml returns RSS 2.0 feed of published notes
- Server-side caching (5 minutes default, configurable)
- ETag generation for conditional requests
- Cache-Control headers for client-side caching
- Configurable feed item limit (50 default)

Configuration:
- FEED_MAX_ITEMS: Maximum items in feed (default: 50)
- FEED_CACHE_SECONDS: Cache duration in seconds (default: 300)

Related: docs/decisions/ADR-014-rss-feed-implementation.md

starpunk/routes/public.py

@@ -5,13 +5,21 @@ Handles public-facing pages including homepage and note permalinks.
 No authentication required for these routes.
 """
 
-from flask import Blueprint, abort, render_template
+import hashlib
+from datetime import datetime, timedelta
+
+from flask import Blueprint, abort, render_template, Response, current_app
 
 from starpunk.notes import list_notes, get_note
+from starpunk.feed import generate_feed
 
 # Create blueprint
 bp = Blueprint("public", __name__)
 
+# Simple in-memory cache for RSS feed
+# Structure: {'xml': str, 'timestamp': datetime, 'etag': str}
+_feed_cache = {"xml": None, "timestamp": None, "etag": None}
+
 
 @bp.route("/")
 def index():
@@ -55,3 +63,85 @@ def note(slug: str):
         abort(404)
 
     return render_template("note.html", note=note_obj)
+
+
+@bp.route("/feed.xml")
+def feed():
+    """
+    RSS 2.0 feed of published notes
+
+    Generates standards-compliant RSS 2.0 feed with server-side caching
+    and ETag support for conditional requests. Cache duration is
+    configurable via FEED_CACHE_SECONDS (default: 300 seconds = 5 minutes).
+
+    Returns:
+        XML response with RSS feed
+
+    Headers:
+        Content-Type: application/rss+xml; charset=utf-8
+        Cache-Control: public, max-age={FEED_CACHE_SECONDS}
+        ETag: MD5 hash of feed content
+
+    Caching Strategy:
+        - Server-side: In-memory cache for configured duration
+        - Client-side: Cache-Control header with max-age
+        - Conditional: ETag support for efficient updates
+
+    Examples:
+        >>> # First request: generates and caches feed
+        >>> response = client.get('/feed.xml')
+        >>> response.status_code
+        200
+        >>> response.headers['Content-Type']
+        'application/rss+xml; charset=utf-8'
+
+        >>> # Subsequent requests within cache window: returns cached feed
+        >>> response = client.get('/feed.xml')
+        >>> response.headers['ETag']
+        'abc123...'
+    """
+    # Get cache duration from config (in seconds)
+    cache_seconds = current_app.config.get("FEED_CACHE_SECONDS", 300)
+    cache_duration = timedelta(seconds=cache_seconds)
+    now = datetime.utcnow()
+
+    # Check if cache is valid
+    if _feed_cache["xml"] and _feed_cache["timestamp"]:
+        cache_age = now - _feed_cache["timestamp"]
+        if cache_age < cache_duration:
+            # Cache is still valid, return cached feed
+            response = Response(
+                _feed_cache["xml"], mimetype="application/rss+xml; charset=utf-8"
+            )
+            response.headers["Cache-Control"] = f"public, max-age={cache_seconds}"
+            response.headers["ETag"] = _feed_cache["etag"]
+            return response
+
+    # Cache expired or empty, generate fresh feed
+    # Get published notes (limit from config)
+    max_items = current_app.config.get("FEED_MAX_ITEMS", 50)
+    notes = list_notes(published_only=True, limit=max_items)
+
+    # Generate RSS feed
+    feed_xml = generate_feed(
+        site_url=current_app.config["SITE_URL"],
+        site_name=current_app.config["SITE_NAME"],
+        site_description=current_app.config.get("SITE_DESCRIPTION", ""),
+        notes=notes,
+        limit=max_items,
+    )
+
+    # Calculate ETag (MD5 hash of feed content)
+    etag = hashlib.md5(feed_xml.encode("utf-8")).hexdigest()
+
+    # Update cache
+    _feed_cache["xml"] = feed_xml
+    _feed_cache["timestamp"] = now
+    _feed_cache["etag"] = etag
+
+    # Return response with appropriate headers
+    response = Response(feed_xml, mimetype="application/rss+xml; charset=utf-8")
+    response.headers["Cache-Control"] = f"public, max-age={cache_seconds}"
+    response.headers["ETag"] = etag
+
+    return response