Replace flowchart/ with a traceable specification in docs/

The two Mermaid diagrams under flowchart/ described the intended flow but
had drifted from the code and carried no link back to it. docs/ now holds
the specification instead: 15 Italian documents (flows, architecture, data
model, API, security, audit log, operations, decisions and limits, bug
history) with 472 numbered requirements, each pointing at the function
that implements it and the test that covers it. docs/processo-di-modifica.md
defines the ID convention and the spec -> code -> test change workflow, and
CLAUDE.md now names docs/ as authoritative.

Writing the spec from the code also corrected stale claims in CLAUDE.md
(B-41's get_history check, BUGS.md still tracking B-71/B-72, Cache-Control
scope, the error contract's bare-string exceptions) and in the operator
guides (payouts do retry, only `drawing` fails to resume, rounds do not
open without fee_address). The B-70 doc test now pins the prose in
docs/scommesse-e-prelievi.md instead of the deleted diagram.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -41,7 +41,7 @@ domain).
 ## Documentation
 
 - [CLAUDE.md](CLAUDE.md) — architecture, commands, domain decisions, known gaps (for anyone/anything working on the code)
-- [flowchart/](flowchart/) — the source-of-truth flow diagrams the implementation follows node-by-node: [platform-overview.mmd](flowchart/platform-overview.mmd) (the 5-phase flow) and [round-lifecycle.mmd](flowchart/round-lifecycle.mmd) (the round/draw lifecycle)
+- [docs/come-funziona.md](docs/come-funziona.md) — the specification (Italian): how the platform works, with diagrams and traceable requirement IDs; it indexes every per-topic document (flows, architecture, data model, API, security, audit log, operations, decisions and limits, bug history). [docs/processo-di-modifica.md](docs/processo-di-modifica.md) describes how a change goes from the spec to the code
 - [docs/setup.md](docs/setup.md) — one-time setup (secrets, master key, migrations)
 - [docs/running-the-server.md](docs/running-the-server.md) — how to launch it with Docker+Caddy (dev vs. production TLS)
 - [docs/guida-utente.md](docs/guida-utente.md) — end-user guide to the test UI (Italian)