feat: overhaul with modular API, agent endpoints, migrations, backup and tests

- Split monolithic app.py into server/api/ routers
- Add Pydantic schemas for input validation
- Add /agent/* API with separate AGENT_TOKEN auth
- Replace static schema.sql with versioned migrations (datenerhaltend)
- Add tools/backup_db.py for SQLite backups
- Add Dockerfile HEALTHCHECK and compose.yml AGENT_TOKEN env
- Add pytest tests for user and agent API
- Update README with API docs and deployment safety notes
- Bump version to 1.2.0

server/schemas.py

@@ -0,0 +1,55 @@
+from __future__ import annotations
+
+from datetime import date
+from enum import Enum
+
+from pydantic import BaseModel, Field, field_validator
+
+
+class LogStatus(str, Enum):
+    taken = "taken"
+    missed = "missed"
+    snoozed = "snoozed"
+
+
+class PinLogin(BaseModel):
+    pin: str = Field(..., min_length=3, max_length=12)
+
+
+class LogEntry(BaseModel):
+    slot_id: str = Field(..., min_length=1, max_length=64)
+    status: LogStatus = LogStatus.taken
+    day: date | None = None
+    logged_at: str | None = None
+    source: str = "app"
+
+
+class SnoozeRequest(BaseModel):
+    slot_id: str = Field(..., min_length=1, max_length=64)
+    minutes: int = Field(...)
+
+    @field_validator("minutes")
+    @classmethod
+    def allowed_snooze(cls, v: int) -> int:
+        if v not in (15, 30):
+            raise ValueError("minutes must be 15 or 30")
+        return v
+
+
+class PushSubscribe(BaseModel):
+    subscription: dict
+
+
+class AgentLogEntry(BaseModel):
+    slot_id: str = Field(..., min_length=1, max_length=64)
+    status: LogStatus = LogStatus.taken
+    day: date | None = None
+    logged_at: str | None = None
+    source: str = "agent"
+
+
+class AgentNotifyRequest(BaseModel):
+    title: str = Field(..., min_length=1, max_length=120)
+    body: str = Field(..., min_length=1, max_length=512)
+    slot_id: str | None = None
+    tag: str | None = None