docs: infer and codify development conventions from codebase

[Copilot]

@MattiaFailla requested a structured, enforceable rule set inferred from the current branch to guide automated code generation and human contributors alike.

Conventions documented (as a PR comment)

• Code style & formatting — ruff ≥ 0.8, Python 3.14+, T | None union syntax, double quotes, mandatory module docstrings, standard import grouping
• Naming patterns — UPPER_CASE string constants, create_<feature>_handlers factories, _handle_<action> private handlers, PascalCase classes, _-prefixed private helpers
• Architecture — three-layer design (handlers → services → db), AsyncRepository abstract base with InMemory/Postgres implementations, closure-based handler injection, all user-facing strings centralized in strings.py, services wired through bot_data (no global singletons)
• Testing — pytest ≥ 8.0, InMemoryRepository for all unit tests (no live DB), per-test instance isolation, @pytest.mark.asyncio for coroutines
• Anti-patterns — no hardcoded strings in handlers, no business logic in handlers, no direct SQL outside repository methods, no Optional[T]/Union[A, B] old-style syntax, no module-level mutable state, no bare except: outside Telegram API call sites

No source files were modified; the rule set was delivered as a reply to the review comment.

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