Add ER diagram and schema docs, enhance tx validation

Added ER.svg and database_schema.md to document the logical data model. Refactored and extended transaction validation in blockchain/abci.go to enforce ID prefix rules, entity existence, and uniqueness for promises, commitments, commiters, and beneficiaries. Updated lbc_sdk dependency version in go.mod and go.sum.

docs/database_schema.md

@@ -0,0 +1,43 @@
+# Логическая модель данных
+
+![[ER.svg]]
+
+<details>
+@startuml
+
+entity Promise {
+  * ID: uuid
+  --
+  * text: text
+  * due: datetime
+  BeneficiaryID: uuid
+  ParentPromiseID: uuid
+}
+
+entity Beneficiary {
+  * ID: uuid
+  --
+  * name: string
+}
+
+entity Commitment {
+  * ID: uuid
+  --
+  PromiseID: int
+  CommiterID: int
+  due: datetime
+}
+
+entity Commiter {
+  * ID: int
+  --
+  * name: string
+}
+
+Commitment }|--|| Promise : belongs to
+Commitment }|--|| Commiter : made by
+Promise }o--|| Beneficiary : has
+Promise }--o Promise : parent of
+
+@enduml
+</details>