Add contribution guidelines, issue and PR templates, and CI workflow

.github/ISSUE_TEMPLATE/bug_report.yml

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+name: Bug Report
+description: Report a bug or broken behavior
+labels: [bug]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Before submitting, search open issues to avoid duplicates.
+
+  - type: textarea
+    id: description
+    attributes:
+      label: Description
+      description: A clear description of the bug.
+    validations:
+      required: true
+
+  - type: textarea
+    id: steps
+    attributes:
+      label: Steps to Reproduce
+      description: Exact steps to reproduce the behavior.
+      placeholder: |
+        1. Go to '...'
+        2. Click on '...'
+        3. See error
+    validations:
+      required: true
+
+  - type: textarea
+    id: expected
+    attributes:
+      label: Expected Behavior
+      description: What you expected to happen.
+    validations:
+      required: true
+
+  - type: textarea
+    id: environment
+    attributes:
+      label: Environment
+      placeholder: |
+        OS: macOS 14 / Windows 11 / Ubuntu 22.04
+        Browser: Chrome 124 / Firefox 125
+        Node.js: 20.x
+        Python: 3.12.x
+    validations:
+      required: true
+
+  - type: textarea
+    id: additional
+    attributes:
+      label: Additional Context
+      description: Screenshots, error logs, or any other relevant information.

.github/ISSUE_TEMPLATE/feature_request.yml

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+name: Feature Request
+description: Suggest a new feature or enhancement
+labels: [feature]
+body:
+  - type: textarea
+    id: problem
+    attributes:
+      label: Problem
+      description: What problem does this feature solve?
+    validations:
+      required: true
+
+  - type: textarea
+    id: solution
+    attributes:
+      label: Proposed Solution
+      description: Describe the feature you want to see implemented.
+    validations:
+      required: true
+
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Alternatives Considered
+      description: Any alternative approaches you have considered.
+
+  - type: textarea
+    id: additional
+    attributes:
+      label: Additional Context
+      description: Mockups, references, or any other relevant context.

.github/ISSUE_TEMPLATE/good_first_issue.yml

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+name: Good First Issue
+description: A beginner-friendly task suitable for new contributors
+labels: [good first issue]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        This issue is intended for contributors new to the project. It should be self-contained and completable without deep knowledge of the codebase.
+
+  - type: textarea
+    id: description
+    attributes:
+      label: Task Description
+      description: A clear description of what needs to be done.
+    validations:
+      required: true
+
+  - type: textarea
+    id: context
+    attributes:
+      label: Context and Background
+      description: Any background information or links to relevant code the contributor needs to get started.
+    validations:
+      required: true
+
+  - type: textarea
+    id: acceptance
+    attributes:
+      label: Acceptance Criteria
+      description: Define what done looks like for this task.
+      placeholder: |
+        - [ ] Criterion 1
+        - [ ] Criterion 2
+    validations:
+      required: true
+
+  - type: textarea
+    id: hints
+    attributes:
+      label: Hints
+      description: Relevant files, functions, or documentation to look at first.

.github/PULL_REQUEST_TEMPLATE.md

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+## Summary
+<!-- What does this PR do? -->
+
+## Motivation
+<!-- Why is this change needed? Link the related issue. Closes #<issue> -->
+
+## Implementation Notes
+<!-- How did you approach the problem? Any trade-offs or decisions worth noting? -->
+
+## Screenshots (if applicable)
+<!-- Before / after screenshots for UI changes -->
+
+## Checklist
+- [ ] `npm run lint` passes
+- [ ] Backend tests pass (`python -m pytest backend/`)
+- [ ] No `.env` files or credentials committed
+- [ ] Branch is up to date with `main`
+- [ ] PR description is complete

.github/workflows/ci.yml

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+name: CI
+
+on:
+  pull_request:
+    branches: [main]
+  push:
+    branches: [main]
+
+jobs:
+  frontend-lint:
+    name: Frontend Lint
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Run ESLint
+        run: npm run lint
+
+  frontend-build:
+    name: Frontend Build
+    runs-on: ubuntu-latest
+    needs: frontend-lint
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Build
+        run: npm run build
+        env:
+          VITE_API_URL: https://karansingh12-freshscan-api.hf.space
+          VITE_DEV_MODE: false
+
+  backend-lint-and-test:
+    name: Backend Lint and Tests
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: backend
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+          cache: pip
+
+      - name: Install dependencies
+        run: |
+          pip install fastapi uvicorn supabase Pillow numpy \
+                      python-dotenv python-multipart httpx pytest
+
+      - name: Lint with Ruff
+        run: |
+          pip install ruff
+          ruff check . --config ruff.toml
+
+      - name: Run tests
+        run: python -m pytest tests/test_ci.py -v
+        env:
+          DEV_BYPASS_AUTH: "false"
+          DEV_BYPASS_TOKEN: ci-test-token
+          SUPABASE_URL: ""
+          SUPABASE_KEY: ""
+          SUPABASE_SERVICE_KEY: ""
+          FRONTEND_URL: http://localhost:5173
+          API_BASE_URL: http://localhost:8000