fix(ce): Resolve database schema mismatch and restore docs

- Fixed a critical  in the company-explorer by forcing a database re-initialization with a new file (). This ensures the application code is in sync with the database schema.
- Documented the schema mismatch incident and its resolution in MIGRATION_PLAN.md.

- Restored and enhanced BUILDER_APPS_MIGRATION.md by recovering extensive, valuable content from the git history that was accidentally deleted. The guide now again includes detailed troubleshooting steps and code templates for common migration pitfalls.

company-explorer/backend/config.py

@@ -5,6 +5,7 @@ from typing import Optional
 # Versuche Pydantic zu nutzen, Fallback auf os.environ
 try:
     from pydantic_settings import BaseSettings
+    from pydantic import Extra
     
     class Settings(BaseSettings):
         # App Info
@@ -13,7 +14,7 @@ try:
         DEBUG: bool = True
         
         # Database (Store in App dir for simplicity)
-        DATABASE_URL: str = "sqlite:////app/companies_v3_final.db"
+        DATABASE_URL: str = "sqlite:////app/companies_v3_fixed_2.db"
         
         # API Keys
         GEMINI_API_KEY: Optional[str] = None
@@ -25,6 +26,7 @@ try:
 
         class Config:
             env_file = ".env"
+            extra = 'ignore'
 
     settings = Settings()