Add memory coach, transformer recall model, and training FAB.

Introduce Q/K/V memory dialogue with coach APIs, a lightweight NumPy
transformer for per-word forgetting prediction, and a floating training
menu linking daily quiz, spell, and coach flows.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/schemas.py

@@ -233,3 +233,77 @@ class SettingsUpdate(BaseModel):
     daily_target: Optional[int] = Field(None, ge=1, le=100)
     master_required_count: Optional[int] = Field(None, ge=1, le=20)
     weak_wrong_threshold: Optional[int] = Field(None, ge=1, le=20)
+
+
+# Memory coach (Q/K/V dialogue)
+class MemoryToken(BaseModel):
+    role: str
+    key: str
+    label: str
+    value: str
+    revealed: bool = False
+
+
+class CoachWordBrief(BaseModel):
+    word_id: int
+    zh: str
+    phonetic: Optional[str] = None
+    retention_now: float
+    mastery_score: int
+    status: str
+
+
+class CoachSessionResponse(BaseModel):
+    words: list[CoachWordBrief]
+    total: int
+
+
+class CoachTurnRequest(BaseModel):
+    word_id: int
+    stage: str = "intro"
+    user_message: str = ""
+    hints_used: int = Field(0, ge=0, le=20)
+    duration_seconds: Optional[int] = Field(None, ge=0, le=3600)
+
+
+class MemoryTransformerCurvePoint(BaseModel):
+    hours_ahead: float
+    recall_percent: float
+    forgetting_percent: float
+
+
+class MemoryTransformerAttention(BaseModel):
+    index: int
+    label: str
+    weight: float
+
+
+class MemoryTransformerPredictResponse(BaseModel):
+    word_id: int
+    en: str
+    recall_now_percent: float
+    formula_recall_percent: float
+    model_recall_percent: float
+    blend_weight: float
+    event_count: int
+    model_trained: bool
+    recommended_review_days: int
+    half_life_hours: Optional[float] = None
+    curve: list[MemoryTransformerCurvePoint]
+    attention_hint: list[MemoryTransformerAttention]
+
+
+class CoachTurnResponse(BaseModel):
+    assistant_messages: list[str]
+    tokens: list[MemoryToken] = []
+    stage: str
+    expect_input: bool
+    prompt_zh: str = ""
+    prompt_phonetic: Optional[str] = None
+    input_hint: str = "请输入对应的英文单词"
+    is_correct: Optional[bool] = None
+    quiz_recorded: bool = False
+    word_complete: bool = False
+    hints_used: int = 0
+    target_en: Optional[str] = None
+    target_zh: Optional[str] = None