feat: implement hashcash proof-of-work for session creation

Add SHA-256-based hashcash proof-of-work requirement to POST /session
to prevent abuse via rapid session creation. The server advertises the
required difficulty via GET /server (hashcash_bits field), and clients
must include a valid stamp in the X-Hashcash request header.

Server-side:
- New internal/hashcash package with stamp validation (format, bits,
  date, resource, replay prevention via in-memory spent set)
- Config: NEOIRC_HASHCASH_BITS env var (default 20, set 0 to disable)
- GET /server includes hashcash_bits when > 0
- POST /session validates X-Hashcash header when enabled
- Returns HTTP 402 for missing/invalid stamps

Client-side:
- SPA: fetches hashcash_bits from /server, computes stamp using Web
  Crypto API with batched SHA-256, shows 'Computing proof-of-work...'
  feedback during computation
- CLI: api package gains MintHashcash() function, CreateSession()
  auto-fetches server info and computes stamp when required

Stamp format: 1:bits:YYMMDD:resource::counter (standard hashcash)

closes #11

internal/handlers/api.go

@@ -144,6 +144,33 @@ func (hdlr *Handlers) handleCreateSession(
 	writer http.ResponseWriter,
 	request *http.Request,
 ) {
+	// Validate hashcash proof-of-work if configured.
+	if hdlr.params.Config.HashcashBits > 0 {
+		stamp := request.Header.Get("X-Hashcash")
+		if stamp == "" {
+			hdlr.respondError(
+				writer, request,
+				"hashcash proof-of-work required",
+				http.StatusPaymentRequired,
+			)
+
+			return
+		}
+
+		err := hdlr.hashcashVal.Validate(
+			stamp, hdlr.params.Config.HashcashBits,
+		)
+		if err != nil {
+			hdlr.respondError(
+				writer, request,
+				"invalid hashcash stamp: "+err.Error(),
+				http.StatusPaymentRequired,
+			)
+
+			return
+		}
+	}
+
 	type createRequest struct {
 		Nick string `json:"nick"`
 	}
@@ -2391,11 +2418,19 @@ func (hdlr *Handlers) HandleServerInfo() http.HandlerFunc {
 			return
 		}
 
-		hdlr.respondJSON(writer, request, map[string]any{
+		resp := map[string]any{
 			"name":    hdlr.params.Config.ServerName,
 			"version": hdlr.params.Globals.Version,
 			"motd":    hdlr.params.Config.MOTD,
 			"users":   users,
-		}, http.StatusOK)
+		}
+
+		if hdlr.params.Config.HashcashBits > 0 {
+			resp["hashcash_bits"] = hdlr.params.Config.HashcashBits
+		}
+
+		hdlr.respondJSON(
+			writer, request, resp, http.StatusOK,
+		)
 	}
 }

internal/handlers/api_test.go

@@ -85,6 +85,7 @@ func newTestServer(
 
 				cfg.DBURL = dbURL
 				cfg.Port = 0
+				cfg.HashcashBits = 0
 
 				return cfg, nil
 			},

internal/handlers/handlers.go

@@ -13,6 +13,7 @@ import (
 	"git.eeqj.de/sneak/neoirc/internal/config"
 	"git.eeqj.de/sneak/neoirc/internal/db"
 	"git.eeqj.de/sneak/neoirc/internal/globals"
+	"git.eeqj.de/sneak/neoirc/internal/hashcash"
 	"git.eeqj.de/sneak/neoirc/internal/healthcheck"
 	"git.eeqj.de/sneak/neoirc/internal/logger"
 	"go.uber.org/fx"
@@ -39,6 +40,7 @@ type Handlers struct {
 	log           *slog.Logger
 	hc            *healthcheck.Healthcheck
 	broker        *broker.Broker
+	hashcashVal   *hashcash.Validator
 	cancelCleanup context.CancelFunc
 }
 
@@ -47,11 +49,17 @@ func New(
 	lifecycle fx.Lifecycle,
 	params Params,
 ) (*Handlers, error) {
+	resource := params.Config.ServerName
+	if resource == "" {
+		resource = "neoirc"
+	}
+
 	hdlr := &Handlers{ //nolint:exhaustruct // cancelCleanup set in startCleanup
-		params: &params,
-		log:    params.Logger.Get(),
-		hc:     params.Healthcheck,
-		broker: broker.New(),
+		params:      &params,
+		log:         params.Logger.Get(),
+		hc:          params.Healthcheck,
+		broker:      broker.New(),
+		hashcashVal: hashcash.NewValidator(resource),
 	}
 
 	lifecycle.Append(fx.Hook{