feat(backend): add database models

backend/src/models/pattern.py

@@ -0,0 +1,53 @@
+from typing import TYPE_CHECKING, Any
+
+from sqlalchemy import Integer, String, Text
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from src.models.base import Base, UUIDMixin
+
+if TYPE_CHECKING:
+    from src.models.question import Question
+
+
+class Pattern(Base, UUIDMixin):
+    """Algorithmic pattern (e.g., Two Pointers, Sliding Window)."""
+
+    __tablename__ = "patterns"
+
+    # Core fields
+    name: Mapped[str] = mapped_column(String(100), unique=True, nullable=False)
+    slug: Mapped[str] = mapped_column(String(100), unique=True, nullable=False)
+    description: Mapped[str | None] = mapped_column(Text, nullable=True)
+    when_to_use: Mapped[str | None] = mapped_column(Text, nullable=True)
+
+    # Tutorial content fields
+    metaphor: Mapped[str | None] = mapped_column(Text, nullable=True)
+    core_concept: Mapped[str | None] = mapped_column(Text, nullable=True)
+    visualization: Mapped[str | None] = mapped_column(Text, nullable=True)
+    code_template: Mapped[str | None] = mapped_column(Text, nullable=True)
+
+    # Structured data fields (JSONB)
+    recognition_signals: Mapped[list[str] | None] = mapped_column(JSONB, nullable=True)
+    common_mistakes: Mapped[list[dict[str, Any]] | None] = mapped_column(JSONB, nullable=True)
+    variations: Mapped[list[dict[str, Any]] | None] = mapped_column(JSONB, nullable=True)
+    related_patterns: Mapped[list[str] | None] = mapped_column(JSONB, nullable=True)
+    prerequisite_patterns: Mapped[list[str] | None] = mapped_column(JSONB, nullable=True)
+
+    # Difficulty level (1-5 learning curve)
+    difficulty_level: Mapped[int | None] = mapped_column(Integer, nullable=True)
+
+    # Pattern classification
+    pattern_type: Mapped[str | None] = mapped_column(String(50), nullable=True)
+    display_order: Mapped[int | None] = mapped_column(Integer, nullable=True)
+
+    # Interactive visualization examples
+    visualization_examples: Mapped[list[dict[str, Any]] | None] = mapped_column(
+        JSONB, nullable=True
+    )
+
+    # Relationships
+    questions: Mapped[list["Question"]] = relationship(
+        secondary="question_patterns",
+        back_populates="patterns",
+    )