feat(blacklist): add per-topic text deny-list with whitelist exceptions

Six public commands let any member of a chat curate two lists of text and
ask whether a given text is blocked: /blacklist_add, /blacklist_del,
/whitelist_add, /whitelist_del, /blacklist_rules and /blacklist_check.

The module is passive. It never reads ordinary chat messages and never
deletes, warns or restricts anyone; the lists stay inert until
/blacklist_check asks about a specific text.

Scope is one forum topic, keyed (Chat.ID, MessageThreadID) and gated on
IsTopicMessage so a reply chain in a plain supergroup does not become its
own unreachable scope. A plain group, a DM and a forum's General topic all
resolve to one chat-wide list.

Matching is substring, after NFKC composition, case folding and whitespace
collapse. Diacritics stay significant, so ma, má and mà are three entries.
A whitelist entry rescues a blacklist match only when it spans that match,
which is what keeps "I met an assassin, dumbass" blocked.

Entry text is percent-encoded before it becomes a storage key, since keys
forbid '/' and cap at 1500 bytes, and is capped at 200 bytes before and
after normalization because NFKC can expand as well as contract.

/blacklist_rules reads each list with Scan, so listing costs one round trip
per list rather than a Get per entry.

docs/blacklist.md

@@ -0,0 +1,86 @@
+# Blacklist
+
+The `blacklist` module lets a chat keep a list of forbidden text and a list of
+exceptions, then ask whether a given text is blocked.
+
+| Command | Parameters | What it does |
+|---|---|---|
+| `/blacklist_add` | `[text...]` | Adds text to the blacklist, or the message you replied to |
+| `/blacklist_del` | `<text...>` | Removes text from the blacklist |
+| `/whitelist_add` | `[text...]` | Adds an exception, or the message you replied to |
+| `/whitelist_del` | `<text...>` | Removes an exception |
+| `/blacklist_rules` | — | Lists both lists in one message |
+| `/blacklist_check` | `<text...>` | Judges a text against both lists |
+
+All are public and single-shot.
+
+## The bot does not police the chat
+
+This is the first thing to know, because the module's name promises something it
+deliberately does not do. Nothing happens automatically. The bot never reads
+ordinary messages, never deletes anything, and never warns or restricts anyone.
+The lists sit inert until `/blacklist_check` asks about a specific text.
+
+Automatic moderation would need three things this bot does not have: a
+message-level hook in the dispatcher, privacy mode disabled in BotFather so
+Telegram delivers ordinary group messages at all, and admin rights with
+permission to delete in every group. All three are out of scope by choice.
+
+## Lists belong to a topic, not to the bot
+
+Each thread keeps its own two lists. In a forum supergroup, entries added in one
+topic are invisible in the next. A non-forum group has one set of lists; a
+private chat with the bot has its own, shared with nobody.
+
+This is the most common surprise: running `/blacklist_check` in the wrong topic
+gives a different answer than the same command one topic over, and it is not a
+bug. Every reply says "in this topic" for that reason.
+
+Within a thread, anyone can add and anyone can remove — including entries
+someone else added. The lists belong to the conversation, so the permission to
+edit them does too. A per-owner rule would strand entries whose author has left
+the group.
+
+## How the whitelist works
+
+The whitelist is not a second independent list. It is an exception layer over
+the blacklist, and it rescues a match only when it **covers** that match in the
+text being checked.
+
+With `ass` blacklisted and `assassin` whitelisted:
+
+| Checked text | Verdict | Why |
+|---|---|---|
+| `assassin` | Allowed | The whitelist entry spans the whole match |
+| `dumbass` | Blacklisted | Nothing whitelisted covers this `ass` |
+| `I met an assassin, dumbass` | **Blacklisted** | The first match is rescued; the second is not |
+
+The third row is the one worth remembering. A whitelist entry does not make a
+whole message safe — it only rescues the occurrences it actually contains.
+
+## Matching
+
+Matching is by substring, after the text is normalized:
+
+- **Case does not matter.** `Cat`, `CAT` and `cat` are one rule.
+- **Spacing does not matter.** `cat  dog` and `cat dog` are one rule, and a
+  newline is just a space.
+- **Diacritics do matter.** `ma`, `má` and `mà` are three separate entries. To
+  catch all three, add all three.
+- **The same word matches across devices.** Vietnamese typed on an iPhone and on
+  an Android phone can differ in how the accents are encoded; normalization
+  resolves that, so the two forms match each other.
+- **Full-width and other presentation variants fold** onto their plain forms.
+
+`/blacklist_check` accepts text of any length. Entries themselves are capped at
+200 bytes — roughly 200 plain letters, or about 65 Vietnamese characters.
+
+## Listing
+
+`/blacklist_rules` prints both lists in one message, each with its entry count,
+showing entries as they were typed rather than in the normalized form. Each
+entry is tappable to copy, ready to paste into a `_del` command.
+
+Telegram caps a message at 4096 characters. A list longer than that is trimmed
+with a count of what was left out; both headings always appear, so a long
+blacklist never hides the whitelist entirely.