feat(ui): add error handling, flash messages and SEO optimization

- Add custom error pages (404, 403, 500) with user-friendly messages
- Add flash message system with signed cookies for security
- Add toast notifications with auto-dismiss and manual close
- Add comprehensive SEO meta tags (description, keywords, OG, Twitter)
- Add canonical URLs for SEO
- Update routes to use slug-based URLs (/posts/{slug} instead of /posts/{id})
- Add Open Graph and Twitter Card meta tags for social sharing
- Add favicon SVG
- Update all templates with proper meta tags and URLs
- Add error handlers registration in main.py
- Add flash middleware for request handling
- Install itsdangerous dependency

app/presentation/web/routes.py

@@ -9,7 +9,7 @@ from typing import Any
 from uuid import uuid4
 
 from fastapi import APIRouter, HTTPException, Request
-from fastapi.responses import HTMLResponse
+from fastapi.responses import HTMLResponse, RedirectResponse
 from fastapi.templating import Jinja2Templates
 
 from app.infrastructure.auth import TokenInfo
@@ -22,6 +22,7 @@ from app.presentation.web.deps import (
     can_see_draft,
     get_user_role,
 )
+from app.presentation.web.flash import flash
 
 router = APIRouter(prefix="/web", tags=["web"])
 templates = Jinja2Templates(directory="app/presentation/templates")
@@ -259,11 +260,11 @@ async def new_post_form(
     )
 
 
-@router.post("/posts/new", response_class=HTMLResponse)
+@router.post("/posts/new")
 async def create_post(
     request: Request,
     user: RequireUserDep,
-) -> HTMLResponse:
+) -> RedirectResponse:
     """Handle new post creation form submission.
 
     Args:
@@ -271,29 +272,17 @@ async def create_post(
         user: Current user (required).
 
     Returns:
-        HTMLResponse redirecting to the home page.
+        RedirectResponse to the new post or home page.
     """
-    context = get_base_context(user)
-    visible_posts = filter_visible_posts(MOCK_POSTS, user)
-
-    return templates.TemplateResponse(
-        request,
-        "pages/index.html",
-        {
-            **context,
-            "posts": visible_posts,
-            "active_page": "home",
-            "current_page": 1,
-            "has_prev": False,
-            "has_next": False,
-        },
-    )
+    flash(request, "Post created successfully!", "success")
+    response = RedirectResponse(url="/web/", status_code=303)
+    return response
 
 
-@router.get("/posts/{post_id}", response_class=HTMLResponse)
+@router.get("/posts/{post_slug}", response_class=HTMLResponse)
 async def post_detail(
     request: Request,
-    post_id: str,
+    post_slug: str,
     user: OptionalUserDep,
 ) -> HTMLResponse:
     """Render a single post detail page.
@@ -309,7 +298,7 @@ async def post_detail(
     Raises:
         HTTPException: If post not found or not visible to user.
     """
-    post = next((p for p in MOCK_POSTS if str(p.id) == post_id), None)
+    post = next((p for p in MOCK_POSTS if p.slug.value == post_slug), None)
 
     if not post:
         raise HTTPException(status_code=404, detail="Post not found")
@@ -332,10 +321,10 @@ async def post_detail(
     )
 
 
-@router.get("/posts/{post_id}/edit", response_class=HTMLResponse)
+@router.get("/posts/{post_slug}/edit", response_class=HTMLResponse)
 async def edit_post_form(
     request: Request,
-    post_id: str,
+    post_slug: str,
     user: RequireUserDep,
 ) -> HTMLResponse:
     """Render the post edit form.
@@ -351,7 +340,7 @@ async def edit_post_form(
     Raises:
         HTTPException: If post not found or user cannot edit it.
     """
-    post = next((p for p in MOCK_POSTS if str(p.id) == post_id), None)
+    post = next((p for p in MOCK_POSTS if p.slug.value == post_slug), None)
 
     if not post:
         raise HTTPException(status_code=404, detail="Post not found")
@@ -373,10 +362,10 @@ async def edit_post_form(
     )
 
 
-@router.post("/posts/{post_id}/edit", response_class=HTMLResponse)
+@router.post("/posts/{post_slug}/edit", response_class=HTMLResponse)
 async def update_post(
     request: Request,
-    post_id: str,
+    post_slug: str,
     user: RequireUserDep,
 ) -> HTMLResponse:
     """Handle post update form submission.
@@ -392,7 +381,7 @@ async def update_post(
     Raises:
         HTTPException: If post not found or user cannot edit it.
     """
-    post = next((p for p in MOCK_POSTS if str(p.id) == post_id), None)
+    post = next((p for p in MOCK_POSTS if p.slug.value == post_slug), None)
 
     if not post:
         raise HTTPException(status_code=404, detail="Post not found")
@@ -415,10 +404,10 @@ async def update_post(
     )
 
 
-@router.post("/posts/{post_id}/delete", response_class=HTMLResponse)
+@router.post("/posts/{post_slug}/delete", response_class=HTMLResponse)
 async def delete_post(
     request: Request,
-    post_id: str,
+    post_slug: str,
     user: RequireUserDep,
 ) -> HTMLResponse:
     """Handle post deletion.
@@ -434,7 +423,7 @@ async def delete_post(
     Raises:
         HTTPException: If post not found or user cannot delete it.
     """
-    post = next((p for p in MOCK_POSTS if str(p.id) == post_id), None)
+    post = next((p for p in MOCK_POSTS if p.slug.value == post_slug), None)
 
     if not post:
         raise HTTPException(status_code=404, detail="Post not found")