feat: Add Go-based feed fetcher to replace Node.js implementation

This commit introduces feedfetcher, a Go program that replicates the feed
fetching logic from the Node.js API server. This is the first step in moving
feed refresh from client-side JavaScript to a background service.

Key features:
- Fetches RSS/Atom feeds via HTTP with conditional GET
- Hashes articles using MD5 of GUID (identical to Node.js)
- Scores articles using Unix timestamp (identical to Node.js)
- Stores articles in Redis sorted sets
- Stores article JSON in S3
- Includes comprehensive unit tests and CI pipeline

Reference: https://github.com/feedreaderco/api/blob/master/src/feeds.js

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: Arpith Siromoney

.github/workflows/feedfetcher-tests.yml

@@ -0,0 +1,82 @@
+name: FeedFetcher Tests
+
+on:
+  pull_request:
+    paths:
+      - 'feedfetcher/**'
+      - '.github/workflows/feedfetcher-tests.yml'
+  push:
+    branches:
+      - main
+    paths:
+      - 'feedfetcher/**'
+
+jobs:
+  test:
+    name: Test
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
+
+    - name: Set up Go
+      uses: actions/setup-go@v4
+      with:
+        go-version: '1.21'
+
+    - name: Download dependencies
+      working-directory: ./feedfetcher
+      run: go mod download
+
+    - name: Run tests
+      working-directory: ./feedfetcher
+      run: go test -v -race -coverprofile=coverage.txt -covermode=atomic ./...
+
+    - name: Upload coverage
+      uses: codecov/codecov-action@v3
+      with:
+        file: ./feedfetcher/coverage.txt
+        flags: feedfetcher
+
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
+
+    - name: Set up Go
+      uses: actions/setup-go@v4
+      with:
+        go-version: '1.21'
+
+    - name: Build binary
+      working-directory: ./feedfetcher
+      run: make build
+
+    - name: Verify binary
+      working-directory: ./feedfetcher
+      run: |
+        ./feedfetcher --help || true
+        file feedfetcher
+
+  lint:
+    name: Lint
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
+
+    - name: Set up Go
+      uses: actions/setup-go@v4
+      with:
+        go-version: '1.21'
+
+    - name: golangci-lint
+      uses: golangci/golangci-lint-action@v3
+      with:
+        version: latest
+        working-directory: ./feedfetcher