tagline: bump max length 260 -> 400, replace magic number with named constant

[vjt]

Summary

• Add TAGLINE_MAX_LEN in inc/tagline.h with an English block comment explaining why the value is not a free parameter (512-byte IRC line cap, send_globops() envelope math).
• Replace the three literal 260 in src/tagline.c (ADD bound, DEL bound, error message) with TAGLINE_MAX_LEN; the user-facing error now formats the constant via %d instead of hardcoding the number in the English string, so the two will never drift.
• Bump the cap from 260 to 350 bytes.

Rationale

Requested on #it-opers by @Hypnotize (2026-04-21): existing limit felt tight and he wanted an increase. The final shape — named constant in the header, block comment documenting the constraint, no raw magic number left — is his direct requirement ("magic number non ne voglio vedere, constant" / "metti un commento in inglese sopra la costante per spiegare che non puoi bumpare arbitrariamente oltre").

Safety / Byte budget

Final value revised from the initial 400 to 350 after review with @Essency, who correctly flagged that the outbound NOTICE wrap in bahamut's send_globops() includes both the services-side *** Global -- from <caller>: ... framing and the ircd-side :<server> NOTICE <dest> :*** Global -- ... envelope. Worst-case (HOSTMAX=63, NICKMAX=32 across caller/through/destnick, bold markers, full fixed tail) lands at ~230 bytes of overhead, leaving ~280 for the text. 350 is past the theoretical worst case for fully-loaded parameters but stays safely under 512 for every realistic Azzurra combination (server name ~16–21, oper nicks ~8–12) — see the block comment above TAGLINE_MAX_LEN for the full derivation and the explicit warning to re-derive both envelopes before bumping again.

Test plan

• Local build clean with make -C src (no warnings introduced).
• Ephemeral testnet smoke — bahamut master + services refs/pull/14/head, 3-server topology (hub + leaf-v4 + leaf-v6 + services):
  • TAGLINE ADD <349-byte string> → Your tagline has been added successfully.
  • TAGLINE ADD <350-byte string> → Your tagline has been added successfully.
  • TAGLINE ADD <351-byte string> → The maximum length for a tagline is 350 characters. Your tagline has 351. (constant + actual length both via %d, no drift).
  • TAGLINE LIST after the run shows the 349 and 350 entries (the 351 was rejected before storage).
  • :hub.azzurra.chat NOTICE testoper :*** Global -- from OperServ: testoper added the following tagline: <350-byte text> GLOBOPS broadcast measured at 454 bytes on the wire, well under the 512 cap.

Smoke harness notes

Auth trick for the testnet: REGISTER → /OPER → IDENTIFY in that order. check_oper() at src/oper.c:603 only promotes past ULEVEL_HOP when user_is_ircop() is already true, so IDENTIFYing before /OPER leaves user->oper == NULL and every ULEVEL_MASTER dispatch returns Access denied. With testoper also configured as M: in services.conf, check_oper() auto-creates the Master oper entry (oper.c:586-591) on first identify — no pre-seeded oper db needed.

=== ADD len=349 (wire line 381) ===
:OperServ!service@azzurra.chat NOTICE testoper :Your tagline has been added successfully.

=== ADD len=350 (wire line 382) ===
:hub.azzurra.chat NOTICE testoper :*** Global -- from OperServ: testoper added the following tagline: A350-xxx...(350 bytes of text)
:OperServ!service@azzurra.chat NOTICE testoper :Your tagline has been added successfully.

=== ADD len=351 (wire line 383) ===
:OperServ!service@azzurra.chat NOTICE testoper :The maximum length for a tagline is 350 characters. Your tagline has 351.

[vjt]

Per request di @Hypnotize su #it-opers: @Essency (Sonic) per code review.

Note: il valore finale nel PR è 350 (non 400). Il block comment in inc/tagline.h documenta il byte budget completo — inbound PRIVMSG via short-form server-to-server prefix + outbound GLOBOPS wrap di bahamut (src/send.c:send_globops → :<server> NOTICE <dest> :*** Global -- ...). A 350 il wire sta comodamente sotto 512 con i params realistici Azzurra (server 21 byte, nick 10-12); truncation teorica solo con NICKMAX=32 × 3 + HOSTMAX=63 × 2 contemporanei, che su questa rete non si presenta.