feat: extract shared infrastructure from shared_lib

Phase 1-3 of decoupling plan:
- Shared DB, models, infrastructure, browser, config, utils
- Event infrastructure (domain_events outbox, bus, processor)
- Structured logging
- Generic container concept (container_type/container_id)
- Alembic migrations for all schema changes

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

browser/app/errors.py

@@ -0,0 +1,126 @@
+from werkzeug.exceptions import HTTPException
+from shared.utils import hx_fragment_request
+
+from quart import (
+    request,
+    render_template,
+    make_response,
+    current_app
+)
+
+from markupsafe import escape
+
+class AppError(ValueError):
+    """
+    Base class for app-level, client-safe errors.
+    Behaves like ValueError so existing except ValueError: still works.
+    """
+    status_code: int = 400
+
+    def __init__(self, message, *, status_code: int | None = None):
+        # Support a single message or a list/tuple of messages
+        if isinstance(message, (list, tuple, set)):
+            self.messages = [str(m) for m in message]
+            msg = self.messages[0] if self.messages else ""
+        else:
+            self.messages = [str(message)]
+            msg = str(message)
+
+        super().__init__(msg)
+
+        if status_code is not None:
+            self.status_code = status_code
+
+
+def errors(app):
+    def _info(e):
+        return {
+            "exception": e,
+            "method": request.method,
+            "url": str(request.url),
+            "base_url": str(request.base_url),
+            "root_path": request.root_path,
+            "path": request.path,
+            "full_path": request.full_path,
+            "endpoint": request.endpoint,
+            "url_rule": str(request.url_rule) if request.url_rule else None,
+            "headers": {k: v for k, v in request.headers.items()
+                        if k.lower().startswith("x-forwarded") or k in ("Host",)},
+        }
+
+    @app.errorhandler(404)
+    async def not_found(e):
+        current_app.logger.warning("404 %s", _info(e))
+        if hx_fragment_request():
+            html = await render_template(
+                "_types/root/exceptions/hx/_.html",
+                errnum='404'
+            )
+        else:
+            html = await render_template(
+                "_types/root/exceptions/_.html",
+                errnum='404',
+            )
+
+        return await make_response(html, 404)
+
+    @app.errorhandler(403)
+    async def not_allowed(e):
+        current_app.logger.warning("403 %s", _info(e))
+        if hx_fragment_request():
+            html = await render_template(
+                "_types/root/exceptions/hx/_.html",
+                errnum='403'
+            )
+        else:
+            html = await render_template(
+                "_types/root/exceptions/_.html",
+                errnum='403',
+            )
+
+        return await make_response(html, 403)
+
+    @app.errorhandler(AppError)
+    async def app_error(e: AppError):
+        # App-level, client-safe errors
+        current_app.logger.info("AppError %s", _info(e))
+        status = getattr(e, "status_code", 400)
+        messages = getattr(e, "messages", [str(e)])
+
+        if request.headers.get("HX-Request") == "true":
+            # Build a little styled <ul><li>...</li></ul> snippet
+            lis = "".join(
+                f"<li>{escape(m)}</li>"
+                for m in messages if m
+            )
+            html = (
+                "<ul class='list-disc pl-5 space-y-1 text-sm text-red-600'>"
+                f"{lis}"
+                "</ul>"
+            )
+            return await make_response(html, status)
+
+        # Non-HTMX: show a nicer page with error messages
+        html = await render_template(
+            "_types/root/exceptions/app_error.html",
+            messages=messages,
+        )
+        return await make_response(html, status)
+
+    @app.errorhandler(Exception)
+    async def error(e):
+        current_app.logger.exception("Exception %s", _info(e))
+
+        status = 500
+        if isinstance(e, HTTPException):
+            status = e.code or 500
+
+        if request.headers.get("HX-Request") == "true":
+            # Generic message for unexpected/untrusted errors
+            return await make_response(
+                "Something went wrong. Please try again.",
+                status,
+            )
+
+        html = await render_template("_types/root/exceptions/error.html")
+        return await make_response(html, status)