feat: add anti-abuse mechanisms

Prevent slot blocking and reservation spam:
- RateLimiter: per-sender cooldowns, hourly limits, pending caps
- ProofOfWork: Hashcash-style PoW for reservation requests
- SenderReputation: track honor rate, no-shows, auto-block
- TherapistPolicy: configurable requirements per provider
- 7 new tests (39 total)

Docs: docs/anti-abuse.md with implementation roadmap

target/debug/deps/meshservice-a34e90bc28566d7f.d

@@ -1,6 +1,6 @@
-/home/c/projects/tool.meshservice/target/debug/deps/meshservice-a34e90bc28566d7f.d: src/lib.rs src/identity.rs src/message.rs src/router.rs src/store.rs src/verification.rs src/services/mod.rs src/services/fapp.rs src/services/housing.rs src/wire.rs src/error.rs
+/home/c/projects/tool.meshservice/target/debug/deps/meshservice-a34e90bc28566d7f.d: src/lib.rs src/identity.rs src/message.rs src/router.rs src/store.rs src/verification.rs src/services/mod.rs src/services/fapp.rs src/services/housing.rs src/wire.rs src/error.rs src/anti_abuse.rs
 
-/home/c/projects/tool.meshservice/target/debug/deps/meshservice-a34e90bc28566d7f: src/lib.rs src/identity.rs src/message.rs src/router.rs src/store.rs src/verification.rs src/services/mod.rs src/services/fapp.rs src/services/housing.rs src/wire.rs src/error.rs
+/home/c/projects/tool.meshservice/target/debug/deps/meshservice-a34e90bc28566d7f: src/lib.rs src/identity.rs src/message.rs src/router.rs src/store.rs src/verification.rs src/services/mod.rs src/services/fapp.rs src/services/housing.rs src/wire.rs src/error.rs src/anti_abuse.rs
 
 src/lib.rs:
 src/identity.rs:
@@ -13,3 +13,4 @@ src/services/fapp.rs:
 src/services/housing.rs:
 src/wire.rs:
 src/error.rs:
+src/anti_abuse.rs: